Ryan Stoughton said:
The main question:
Which one's better for a campaign, and why?
Both are equally suitable for long term campaigning. The difference is that if using the OD&D books, Vol I-III, you might have to use the supplements, Dragon and Strategic Review Articles (found the the Dragon archive CD Rom), other sources like Judges Guild products, and some well crafted house rules to get the feel of the game you want. I say might because I am putting together a OD&D game based solely on the three little books, with a few of my house rules thrown in. Knowing your involvement with Microlite 20, you could pull this off if you want to. The Rules Cyclopedia is an amazing book, and complete with lots of optional rules. There are way too many optional rules for my taste, but a lot of folks I have met that didn't have actual gaming experience with the older editions of D&D, and that tried 3rd edition and didn't care for it for whatever reasons, seemed to really love the RC. I think the reason is, and this is completely for another thread, is that the Rules Cyc. is as much (if not more) an influence on 3rd edition D&D than 2nd edition was. I won't derail this thread with that topic though.
Extra Questions:
Can stat blocks fit on playing cards?
Yes to both, although I think with RC D&D you might want a little bigger card, just cause of the more info available.
How much is NOT d20 roll high?
If I read your question correct, in OD&D I cannot think of any instance where to rolling of a d20 with the intention of rolling low is the goal. In RC D&D, the only instance I can off the top of my head remember is when using the
optional ability check (when you roll a d20 and it has to be equal or less than the relevant ability score.) I don't use this method. There are times when rolling low is good like morale but the dice are different, (2d6 for morale.) Saving Throws are always roll the highest possible on a d20, as are attacks.
How does it handle player characters of different levels?
Very well, but the edge on this one goes to OD&D. In Vol I, everyone uses a d6 for hit dice so the spread isn't that big. Even if using Supplement I Greyhawk with different hit dice types for each class, the range is not that great that PCs of different levels won't be out of sorts. Keep in mind by default, in either edition, hit points are rolled and not given. So you might have a 3rd level fighter with 9 HP and a 2nd level magic user with 8 HP due to some lucking HD rolling.
What's the sweet spot range?
I say with every pre-modern edition of D&D, the sweet spot starts at 5th level, and as cheesy as it sounds, because that is where not only does the survivability increase dramatically , but the most sought after spells become available, (i.e., lighting bolt and fireball to name two.) It ends around 9-12th level depending on which pre-modern version D&D you are playing.
How does it handle player characters of different levels across the sweet spot?
See above. Pretty much the same.