It sounds like you must already have a computer if you are thinking maybe a sound card. If you go the computer/soundcard route, you have many many possibilities for upgrades, expansions,changes,etc. A computer setup is probably the cheapest way to go in the longrun.
You say you dont want to upgrade for a while,but by going computer, you leave that option open . The portable systems like the Roland VS series are great little units for portable gigs and they work very well for home recording too, but ,as I see it, they are stronger with audio capabilities than with midi ones. If you get good software, your midi possibilities increase manyfold. Yahmaha has a new portable unit coming out that looks to blow the pants off of Rolands unit, but still for midi, I would always recommend a computer.
Another plus in the computer camp is that for midi, you dont need the fastest cpu,or the most memory. You can record in midi,and when you are finished, mix it all down to a few audio tracks(a real memory saver).