Strangely enough, i have both of these guitars!
Basically, they are VERY different guitars. Now i know that technically they shouldn't be - same scale length etc... but somehow, whilst the RG2550E feels and sounds a bit cold and clinical - very 'technical' (very thin, fast neck) - the RGA121 is very warm, both in terms of tone and playability.
My RG2550E is a 2004 model, and the neck has a slightly gloss sheen to it - where the RGA's neck is a very smooth satin finish. I'm not sure if this year's 2550Es have the smooth finish of the RGA (the RG2620CBL i tried recently has it), but personally i prefer this to slightly shiny neck on my 2550E.
If it's a classic true RG-feeling guitar that you want, the RG2550E is definately in that category. It feels very precise and cutting - if you're going to be shredding mad fast metal style, then it's probably the one to go for. I dunno if it's the double-locking system, but it feels 'tight' if you know what i mean(!?) - i couldn't imagine many blues players using it, but it's ideal for stuff like Megadeth or Slayer or something.
The RGA feels a lot more like a worn-in vintage telecaster, bizarrely enough... but with a super-smooth, fast neck - and the RG scale length. I totally love mine. It's a joy to play, very versatile, and feels a bit more 'loose', a bit more alive, really. The carved top is really nice too - makes it feel more like an 'S' model... It's got a kind of 'springy' playability that most RGs i've tried don't have. But if you're just going to playing metal, despite what ibanez's literature says, the RGA to me seems far less suited to it than the RG2550E.
The stock pickups in both guitars weren't/aren't to my tastes - it's a fairly safe bet you're gonna want to change them for something a bit more specialised (i stuck some seymours - a JB and a Jazz - in my RGA and it sounds sublime) - I'd say though that the dimarzio/ibz pickups in the RG2550 are probably better than the V7/V8 that come with the RGA... a bit less muddy, a bit brighter and cleaner. Plus you get the single coil. But as lyconxero rightly points out, it's all subjective.
Hope this helps... Sorry if i rambled a bit, haha! the best idea really is to get down to a music store that has them and try them for yourself - see which you like better!
Oh, and a pic: