In my experience, there is NO comparison.
EXS was a sample player that could cope with the VSL library... it was great in it's time - I changed to Logic from Cubase rather than go the PC/Giga route, or use Kontact (less efficient at the time) - respect to EXS in it's day...
However for VSL NOTHING beats the Vienna Instrument. It's another world apart from having to manually adjust your repetitions, and the way you can just drag and drop articulations is something you don't fully appreciate until you've used it a little bit - even the videos (which I encourage you to check out if you haven't done so already) don't show fully how much fun this instrument is to use. If you've got a good idea, and a reasonably fast computer, it's so efficient and quick (relatively speaking to the complexity of the preset/matrix) you have a short load time from you hard drive but apart from that, you can preview sounds so well - for example, load a legato violin, then you want to see how the viola soudns on the same section, you try it, prefer the violin, when you load the violin back again, it's basically already remember in memory, so takes just a couple of seconds (instead of 10). The Pro Ed required a lot of patience, a steep learning curve for many, and ultimately the results could usually not come close to the VI product that is easy to impliment, fast to set up, efficient and much more flexible. There are just so many features simple yet ingenius - doesn't compare to EXS.
Another way of putting it, I would give the VSL Pro Ed +EXS combo a 4 out of 10 for usability, only because VSL sounds so great etc etc, but I would give the Vienna Instrument a 10 out of 10 by comparison.
Another way of putting it yet again, a project that might take me all day in Pro Ed with manual tweaking of repetitions and the like would by comparison only take me an hour in VI, it's THAT fast and efficient, and this easy of use really helps towards preserving your inspiration, it lets things flow more and you don't get stuck in technical things not even half the time, practically not at all, at least that is what I've found. The worst you have to do, which is actually one of the best features, is to optimise RAM - but if you use Cubase as I just found out you don't need to do that if you freeze the track - you can unload the instrument which free's up the RAM AND the cpu... very cool. If you use logic, the freeze function presently keeps the samples in ram, so when using Logic I optimise and then freeze but it's painless enough, and best of all you can just keep on loading instruments.
So, for Pro Ed, I could load say for example 20 articulations including a couple of legato patches, and that was it (unless I bounced to audio which I didn't usually have the patience for!) but with VI and the optimise feature + freezing I can load 10 times that amount on the same machine with the same RAM and cpu.
Hope that helps. yes this is a glowing review but deservedly so, you can't compare EXS and VI. Don't forget also VI is the native bitrate of the library - 24bit, and yes, over a mix IMO you absolutely hear a HUGE difference between the 16bit and 24bit. For me I relegated my Pro Ed library to a backup drive that isn't even plugged in anymore, I just don't need it. VI has all the samples of the Pro Ed plus many more in a format hugely more easy to access and use and enjoy.
EDIT: Oh yes the accessing of the editor is an issue as described in the above post - however consider the technical limiations of host software - as I understand it this was the best way to integrate a an instrument that would be able to use it's own sample "server" as the VI does, so it was a compromise, however considering the other time "savers" I would say this saves you 8 hours in the day and loses you 20 minutes - overall, a big save!
Miklos.