I agree with the above posts. It does depend on what you want to do and, nothing will, at this point, substitute for a good workstation. You should start with a good workstation and then add soft synths for two reasons. First, before you get to the latency, there is the controller that you will need. Therefore, since you will need a controller and sounds, you may as well purchase a workstation. Workstations also help you to do things a bit quicker and you will get a large variety or readily available sounds.
Second, and actually, somewhat related, soft synths are combersome at times, require setup, have latency, can crash you computer at the worst possible moments and, really tax you CPU power and thus may greatly reduce track count. If you want to emulate an orchestra, you will need a fairly high track count.
Many pros with the csh and busy schedules have seperate computers for thier soft synths, usually a Gigastudio or two. This is similar to sample tank however Giga only works on PC and Sample Tank can work on Mac. Giga has the decidedly better samples availbel in its format though.
If I were purchasing today, I would most definetly choose betwwen a Triton workstation or a Motif workstation depending on which suited my needs, then add soft synths and soft samplers,
So, with that said, recently, I was with a well known keyboardist whom you most certainly have heard on many well known recordings. He recently sold his Motif workstation, purchased the module and is using a controller for all modules and soft synths. That is certainly possible and would be workable, however, you should consider that the rest of the room contained at least $100,000.00 worth of ourboard gear of ever pursasion.
Oh yeah, one other thing, workstations such as the Triton and Motif have built in effects which can be and are very useful and important.
In the final analysis, it is never, every the euipment you have but what you do with it. Remeber, too much emphasis is placed on the technology and not enought on the artistry but all too many folks.
If you can get the sound you want without spending much, then go for it. Also, an interesting quote I recently read though I am paraphrasing since I do not recall the exact quote or who actually made it but, the quote was that it was better to really know one instrument that to have an arsenal of keys and modules that you do not quite know how to work all that well.
So, you see, choose the instrument that gives you most of what you want in a self contained package, learn the shit out of the thing and you will probably be better off than someone with a score or more of modules, soft synths, samplers and what have you that has only a cursory knowledge of how they all work.
The end.
