My Bass player loves the Ampeg Versus the Mark.  I have both and use Ampeg nearly always.

you guys are making curious. i reckon i'll need to check out the mark bass myself, but too much to do and not enough time at the moment ..

I just checked to see if IK Ampeg SVX works here, in case my mind was playing tricks on me, and it's fine.
 
I still use Amplitube more nowadays, though.

I think you would be good with either (or both!!!!!!)

I have Ampeg SVX and use it all the time in X2 via the free Amplitube 3 plugin - never had any issues 

The bass player in my band uses Mark Bass gear for gigs and we recently recorded in the studio

The Mark Bass plugin did everything he wanted and he managed to get "that sound" really easily so I think you will be fine

Don't forget about GK Amplification.  Audiffex just announced a permanent price drop for both the standard ($39) and the pro ($79) versions.  Gallien-Krueger bass vst is clear, punchy and has a big tight bottom.  It is my favorite bass vst.  http://www.audiffex.com/EN/gallien.html

Don't know about the others, because I stopped my search when I found IK's SVT.

My primary instrument in bands has always been bass. I own a 1981 Ampeg SVT with 8 10's. Now, I just rent it out to classic rock bands traveling through town who need backline. IK has NAILED the sound. 
 
I've never had a problem using it with any version of Sonar I've owned. If you need flexibility, get Amplitube Free, demo the other bass rigs and just buy the one you like best. They have several great ones.

If you own svx you can open the models in the amplitube "free" version or custom shop so it will be a native 64 bit vst. I have given thought to picking up mark studio. Honestly I use the GK model in amplitube using a 410 cab and the green (trace elliot) model. Sometimes the ampeg models depending on the song/tone I am after.

I like using Amplitube Fender bass heads/cabs better than the Ampeg models but I don't do metal or hard driving rock. As an alternative for pop/indie/country/jazz bass sounds I prefer something quite different - Nomad Factory Magma which gives a ton of rack effects with very flexible routing. 3s

I have both and Mark Studio is the clear winner IMO.  SVX is quite good but for focused, fat sounds that really work in a mix Mark Studio is the best.  I use it with mu Fender Precision and various NI Bass Samples.

Mark Bass is excellent, there are such a wide variety of bass sounds you can get from it, definitely versatile. The presets are great starting off points. I find it's also great for synth basses. I don't use the effects pedals much, except maybe the distortion which is great for dialing in just a little bit of fuzz to make the bass pop.