Both image very well (and similarly), both are comfortable, both are very good with gaming and electronica. The 900 have more bass, offer isolation, but have somewhat withdrawn mids; the 2900 sound a bit more open, have less bass, and more present mids. Really either is going to be a good candidate. Price-wise, at current prices (they're actually the same list price), the 900 are probably a better "value" (I hate discussions of value though) - simply because they're ~$200 less via Amazon today. For just gaming, unless you have both to sit and A/B, I'm not sure you'd really notice a difference - both will be very good (especially compared to most of the "gamer headphone" garbage that's been put out over the years). For music listening, the 2900 have a bit sweeter voicing, but at the expense of bass slam (don't get me wrong, both are fairly bassy headphones). For electronic music this different isn't a huge problem imho, but for some genres the 2900 will probably have an edge.
Finally you must remember that the 900 and 2900 are both fairly unforgiving and bright headphones - nails driven on demand. If you listen to a lot of poorly mastered, heavily compressed, etc audio, the HFI-2400 would be a MUCH better suggestion (for gaming they're going to be just as good as the more expensive PROline models, but they're more forgiving, which will lend to listening to a wider variety of music - honestly you don't need high-end cans for quality gaming audio).
So the easy answer is "either" unless you're concerned about isolation (imho it's a good thing for gaming), or "neither" if you're intending to feed low quality media into them.
Honestly I'm more inclined to agree with Headphonia than the "rule of hype" that says the 900 are "bass and nothing else - bass!" While they aren't my favorite headphone for swing, jazz, or rock - they aren't slouches (and my primary complaints are similar to my complaints regarding the 2900 - they're somewhat dry and aggressive, and relatively unforgiving). The extra bass isn't mud-and-blood, and doesn't push itself in where unwanted; they get a bum rap as being good for nothing but modern bass-blast genres simply because they have good bottom-end extension and impact.