Both games have the traditional Bomberman battle mode, the quality of which is about equal. The big difference comes in the additional battle modes. Each game has 4 or 5 new variations on the old battle style, and in Bomberman Generation most of them are actually really great - more fun than the traditional battle mode in ways. One of them, Revenge Battle, has you firing bombs from the sidelines at worms that pop out of the ground, trying to bounce them across the heads of as many worms as possible to build up big combos - it's like a more fleshed-out version of a Mario Party mini-game. Another, dodge battle, puts you in an arena with bombs raining down on you from above - all you can do is run away or punch and kick the bombs, so it's a frantic scramble to both avoid getting blown up and to knock the randomly falling bombs towards your opponents. Still another, Reversi Battle, is a battle to 'paint' as much of the arena with your bomb blasts. Staying alive is still important, though, as dying makes you lose half of the panels that you've claimed. At the end of the round, the bomber with the most panels claimed wins.
The main difference with Bomberman Jetters, as far as I can tell, is that the new battle modes just aren't that great. Rather than bring back the great new variations from Generation, they opted to come up with a whole new set. One of the good ones is Smash Bros-esque - rather than kill your opponents with bomb blasts, you try to knock them off of the board altogether. Another cool one is Flicky-esque. You have to collect as many balloons as you can, with each balloon attaching itself behind you in an ever-grown string. However, to get points for the balloons you have to deliver them to a gate and cash them in. The rub is that the more balloons trailing your bomber, the slower he moves, and the easier for another player to run up and bomb your balloon chain to break it off.
Another addition in Bomberman Jetters is the "killer shot," a super move that your bomber can execute after setting a certain number of bombs. Killer shots are character-specific, so it finally matters a little bit which bomber you decide to use.
Regardless of the additions in Jetters, I still think Generation is by far the better game. It might not sound like it from my descriptions, but the battle modes are much, much more fun. It also has far better music, which I actually did a rip of in case you're interested in hearing any of it.
Really though, my advice is pretty predictable - buy both. At $20 each they're totally worth it. Both have nice full-color manuals to boot!
