Can do
for bass: Ion. The filter modules actually add frequency content to the sound. It's just really quite a bit bassier in general. The virus I have, the virus kb, doesn't seem to be on par in that regard, although it's from my experience better for more non obtrusive bass such as that used in genres like EBM or Psytrance.
for strings: Virus. It's just seemingly a lot easier to get huge string sounds on it due to the ion lacking stereo spread in unison mode and the virus having a LOT more onboard FX. Both have the required oscillators/filters, but it's much easier on the virus.
for leads: Tough call. For leads that are going to stand out themselves in the mix, usually the Virus due to it's aggressive sound. For slower tracks that need "bigger" sounds, I find the Ion to be superior. Both are good at sync leads and whatnot, but I'd give the ion the edge in PWM based sounds. The virus though just has this quality to it's high passed and band pass filters that I love, seeing as I use those a lot, and it seemingly cuts through the mix very easy.
general mayhem: You can't go wrong with either one, but I think the Ion wins here. Huuuge mod matrix, crazy sequencer thing, tracking generator,and a superior control interface. The knobs are extremely solid in comparison and high resolution. As in, you can tweak the filter just like one of those old synths and the synth doesn't use the zippy 127 step deal a lot of V/A's use.
The virus in comparison seems easier to work with LFO wise and has a few of the more useful options not buried in menus. Not that there's TOO many of those on the Ion. Generally, you press a button, and said options move to the huge screen in the middle for your editing needs, all controlled by buttons and a very well placed knob.
The virus has a much, much better arpeggiator. The Ion only allows for a random pattern or one of the factory patterns for some reason, while the virus not only has it's own patterns but all the other standard arp options. The Ion has an easier to implement multi mode, while the virus has an easier method of routing filters.
Hard to choose which one I like more. Glad I own both. After I get a Blofeld and build the modular I want along with the analogs I already have i'll be set sound wise.
TL;DR-Alesis Ion is a very versatile, great sounding synth. As is the virus, and both are good for different things.
Try and find one, and don't get the Micron, as the micron is particularly annoying to program.