I should start with a brief history of my experience w/ music games:  I wasn't really on board until two summers ago when Guitar Hero 3 went on sale for $25 w/ the Guitar. I was surprised by how fun it was and later picked up Guitar Hero 5 (Which has a set list I love) - and eventually Rock Band: The Beatles.
Obviously RB:TB is all Beatles all the time but the note highway seems off to me - fewer notes (which may be symptom of their simple song structure)  but more touchy about the timing. It seems that most people like Rock Band better - and based on my experience w/ RB:TB I can't see why - is RB2 a notable improvement over RB:TB ?(I know it came out 1st but you get what I mean). 
All of this kind of leads towards this: I want to expand my plastic band set up to include a drum kit - I've read all sorts of stuff saying that the GH ones are crap (they're also double the price of an RB2 kit) but I'm concerned about cross-platform compatibility - I don't want to buy a RB2 kit just to have it not work w/ GH5 (and then either have to switch to RB and waste my GH DLC or buy the GH Drum Kit). So which drum kit is best? (Yea, I've read about the $200 Ion Kit - not happening) How does it work when being used with the other game (Both GH Drums to RB2 and vice versa).

I would definitely get a copy of Rock Band 2. There's so much DLC it's untrue and and the career mode is much more interesting. Plus, I prefer RB charting techniques.
 
Drums wise, consult this chart: http://www.rockband.com/compatibility
If you're on Wii, it looks like a Guitar Hero kit is your best bet as they are compatible with both Guitar Hero and Rock Band games. 
If you're on 360 or PS3, I would get a Rock Band kit as they're cheaper, as you said, and I found them to be more comfortable on both games. Using a Guitar Hero kit on Rock Band is kinda awkward than using a RB kit on GH.