If you're going to get BIAB, get the megapack, as it has a far better selection of styles. Yes it sounds like a 10 year old kid's casio keyboard if you use a Soundblaster (aarrrghh!) and the new included Roland VSC is disappointing. But play it through a SD-20 or a software synth and you'll be pleasantly surprised.
In fact the Cakewalk TTS-1 in Sonar works pretty good with it - as good or better than the Coyote Forte Dxi which they try to sell you on their website. Sampletank as a plug-in doesn't work properly, but export the BIAB as a midi into Sonar and just change the instruments with Sampletank and you'll also get a good sound.
I think Jammer has potential, and more easily customizable, but the last time I checked, it didn't have VST/Dxi compatibility, so you'd have to again use the export midi to Sonar, or use a hard or soft synth. It definitely has a different feel and you could use both if you had the budget. Sure it ain't professional but many of us here are hobbyists and they both can produce great results that couldn't be achieved without access to a hoard of musicians.
All in all I'd vote for BIAB by a narrow margin.