Rob,
The SB 2 would be the best choice if you want to get the analog section modded. I can fit the rather large Sonicap coupling caps in that case. With the SB3, I have to use Black Gate NP electrolytics bypassed with a small Sonicap Gen 2.
If you get a "Full" mod you can get both the digital and analog section modded.
I have yet to hear an outboard DAC that sounds better the a full modded SB2 with Bybees and Teflon caps.
Of course, we are talking close to $1,000.00 worth of mods at that point.
The SB3 digital mod with a Silver Bybee Purifier is $375.00. If you already have a DAC you are happy with or are running into a Tact, that would be a "Best Buy". If you do not have a DAC, I would really suggest you take the money you planned to spend on the DAC, power cable and digital cable and put it into more mods to the SB 2.