If you're looking for software that does the same basic thing that Jam Station does, two options are Band-In-A-Box and Jammer. These programs provide numerous musical styles and the ability to input whatever chord progressions you want.
Jammer provides more detailed control over the "studio musicians" you're playing with (how funky our how straight they play, etc.), which can really improve the feel of the groove, but Jammer is more difficult to master.
Band-In-A-Box is very easy to use. I do demos for several songwriters and I often start with band in a box to get my basic rhythem tracks (keys, bass and drums) together and then add guitar and vocals. We've actually sold some songs to publishers which were recorded this way.
As a result, a couple of the songwriters bought Band-In-A-Box and now prepare the base tracks at home and then bring a disk to my studio to add guitar and vocals, etc. These are not hot players (they are decent songwriters) but with this software they can create pretty convincing arrangements.
The main issue with both of these programs are - the sofeware provides a MIDI performance which must then be sent to a synth or a module to actually generate the sounds. This means you not only buy software but at least one decent keyboard and/or module.
As you know the Jam Station has not only performances but also sounds built in. It is also more portable than a computer/module system.
You could go with something like ACID sofware, which provides audio loops. I've not used ACID so I don't know how much control you have over programming specific chord progressions, etc. I'm sure there are several threads on this site regarding ACID. In fact there is a Sonic Foundry "chat room" on this site.
The computer route makes much sense if you are looking to record. And it is likely that once you start playing to sequences you will eventually create songs you want to record. However, If you are convinced that all you need is a box to provide some tracks to jam with, Jam Station may be less complex (and more portable to jam with).
For the record I have a Jam Station (which I have a blast jamming with). I also have Band-In-A-Box and Jammer which I use for certain recording applications (into Cakewalk). I consider them to be different tools for different jobs.
Good luck in your decision.