I've DM'ed both. Ran a BECMI campaign for over 10 years, and basically ignored AD&D and 2e until the Planescape campaign setting came out.
"Old" Immortals (gold-covered books) was, for want of a better term, somewhat broken. Unless the DM has placed the PCs in a constrained setting (say, a plane where magic or power/ability attacks don't work), even low-level immortals can pull off some truly jaw-dropping stuff.
Each PC has basically five different attack forms (weapons, magic, ability attacks, power attacks, and... can't remember the other one, off the top of my head). Most Immortal-level monsters are immune to at least one of those forms, but are often heinously weak against another form. This leads to very swingy battles. It also leads to some instant-kill issues, where a creature of godlike power can be cut down in a round or two because (example) several opponents are all hammering it's Intelligence, or power points.
Character creation options are rather generic as well. Wrath of the Immortals (WotI) introduced domain powers (kind of like god-level feats) for immortal PCs, which helped differentiate characters more. Otherwise, a level 1 Temporal immortal is much the same as any other level 1 Temporal immortal, except for base stats and Sphere.
Demons were hideously underpowered in gold-cover Immortals, which is a real shame because they made for some of the more interesting enemies. I mean, there are only so many enagegements you can have with proteans and flickers before you start saying: "Oh good... another ooze... and another flashing light-thing". Fortunately, WotI fixed the demons as well - or at least improved them.
As soon as Wrath of the Immortals came out, we switched over. It simplified things hugely, provided better balance in combat, removed or nerfed some of the near- instant-kill combat options, and it provided a really nifty set of NPC immortals to interact with (including personalities and motivations).
As far as the modules go, my favorite is probably IM1. It's broken into several parts which involve puzzle-solving, outright combat, and exploration at an Immortal level. There's a nifty segment where the PCs end up on modern Earth, which is reminiscent of the classic Dragon magazine module (issue 100) called "The City Beyond the Gate". Uber-powerful Immortal avatars, minus their spell powers, vs "learning how to drive a car" and SWAT teams with assault rifles. Good times.
IM2 is a little more involved and plot/NPC -driven than IM1. IM3 is a bit of a mind-bender. It can get downright silly at times, too... and the silly tone is not helped by the internal illustrations within the module.
