I am a huge fan of Deadlands. I have only played the Classic version, but I own Deadlands Reloaded and D20, and I have used the Savage Worlds system for other games. I will try to hit the high points, although I could easily add much more information on them. I will try to explain system references, but it is helpful to read the wikipedia entries to get a baseline for some things.
DEADLANDS CLASSIC:
Pros:
There is a lot of customization available. Characters of different archetypes feel very different.
Dice pools for stats and skills mean less variability in character competence (i.e. it is less likely for a highly skilled character to fail low target numbers or to botch). As mentioned by others, dice pools are generally 1-5, so you are not rolling so many that it is difficult to keep track of.
There are a lot of supplements available for support.
The system is extremely flavorful (the ones that immediately come to mind are that playing cards are used for initiative, poker chips are drawn randomly at the start of each session and are used to add to die rolls or can be converted into experience or to can buy off wounds, casting magic and creating "weird science" devices involves drawing poker hands).
The system and setting come together very well (i.e. the system inherently encourages the tropes of the genre).
It can use many different die types, so you get to feel like you're using a lot of your dice.
Many enemies have flavorful weaknesses that the party can figure out to make defeating them much easier.
The system has fairly simple core mechanics.
The books are very flavorful and fun to read (being largely written with a western, conversational tone).
The game can handle several different genres well, including spaghetti western, steampunk, mystic, and horror, or combinations of the three.
Cons:
Acquiring all of the supplements can get expensive if you are a completist.
There are a number of sub-systems that can get complicated, and it can be hard to find some minor rule.
There can be wide variation in character's combat (and non-combat) capabilities.
Some enemies can only be defeated if the party knows their weakness.
Other:
I have heard people say that combat can take awhile. This is naturally relative, and also depends on your group. I have found that combats with the GM using "mook rules" for minor enemies, and where the players are familiar with the system and are ready to go when their turn comes up, can run half an hour to an hour. If the GM is using the full rules (i.e. every enemy can take 5 wounds in all locations, hit location is rolled randomly) for every enemy, and/or the players are slow to decide what to do or are very unfamiliar with the rules, a combat can run up to 2 hours.
Supernatural enemies often have particular weaknesses that either make killing them easier, or are required in order to kill them. This can be good as it makes non-combat skills more useful and allows for fun roleplaying and investigation, as well as the benefits of puzzle solving. It can be bad if the players just can't figure out the weakness, or if they just want to shoot something.
DEADLANDS D20:
Pros:
It uses the D20 system, which you and your players are familiar with. This also means that OGL content may be useful for the game.
It has the excellent background of Deadlands, and can handle the same genres.
Cons:
The system is generic, and is modified to fit the setting.
Much of the flavor is only as a background, and the system does not particularly support the specific flavor of the setting very well.
DEADLANDS RELOADED (SAVAGE WORLDS):
Pros:
The system is relatively simple and easy to grok.
It is easy and not particularly expensive to get the core books for the game.
There is pdf support for the game.
The system is still actively supported, so more supplements may come out.
The system is very good for the spaghetti western and steampunk genres.
Combats run a bit more quickly than in the other two systems.
The system and setting mesh well together, as the Savage Worlds system is based around simplifying the Deadlands system.
There are many generic Savage Worlds supplements that can be used to add new powers and other material to the game.
"Bennies" act as fate points to allow the characters to shine when they want to.
Cons:
Many of the powers are generic, and so it may feel as though there is less difference between characters (for example, a mad scientist's gizmo, an indian shaman, a priest, and a spellcaster all use the same power, just with different flavor).
The system can seem a bit random, as the player is only rolling two dice (their main die and a d6) and taking the highest number. This means that botching is more common, and a very skilled character will fail basic rolls more often than in classic deadlands. It also means that there is a large jump in difficulty between difficulty numers that are 7 or lower and those that are above 7.
The system does not, imho, handle the horror aspects of the game as well as Classic did.
If you're still reading this, I am also familiar with Earthdawn, although I have not had an opportunity to play it. It is currently in its 3rd Edition, and is published by Redbrick Limited. Many of the 2nd Edition books were compiled into the Gamemaster's Compendium and the Player's Compendium (each are massive tomes), which are available via lulu.com. The 3rd Edition has several core rulebooks and supplements already out. The system is largely similar to the 2nd Edition, but has had some significant changes.
Earthdawn is a fairly complex system, about as much as D&D, although in different ways. The system and setting work well together. You can get a lot of background info from the Redbrick web site. You can also check out rpg.net reviews and wikipedia to get a feel for the game. The setting has a lot to recommend it. I personally like that the racial mix is different from standard fantasy without having too many or departing far from normal fantasy tropes (in addition to humans, elves, and dwarves, there are trolls, small winged fairies, rock men, and lizardmen as major races). There is a wide variation in levels of technology and information, and large cultural differences in the world. There is a well developed villain (the Theran Empire and their slavers) that is not completely evil. Overall I like Earthdawn, although like many other games I like I am not sure I will have the chance, as there are so many demands on my time and only so many games I can get people interested in.
I also prefer the Huckster's magic in the original, it felt much more like a quick round of cards with the Devil when casting your spells.
