I currently own one of each of the UL310's. The main diff between the two is the B&C drivers have a strong mid bump around 380Hz, which means, with no amp EQ, they hit you in the chest more.
The custom Eminence drivers seem to have a wider frequency response, lower lows, and higher highs, but hooked up to the same amp, the S1 310 will "seem" louder. The S2 cab takes EQ VERY nicely, however, so if you want to see what the S1 sounds like if you have an S2, just bump the low mids. I'm told these Eminence drivers are custom for Epifani, but to be honest, they sound very much like the standard Deltalite II's to my ears. Not a bad thing, as I like those speakers and have used many of them. Just a note.
Either one is very capable at handling the horsepower, as well. They're rated at 750W (3x250W drivers) and I'd feel confident pushing them.
How I ended up with one of each is a long story, part of which was my own curiosity between the two's diffs, but at least at this point, I'm kinda' diggin' having one of each. As a stack, they work together really well. The only issue is that odd impedence, so you have to use a 2 ohm capable head or a preamp into a stereo power amp rig to drive both.
This past weekend, I ran my Eden Time Traveler (WT405) direct into them as well as used it as a preamp into my Stewart World 2.1 . The former was very impressive, but the latter had to be heard to be believed.
On a small outdoor gig, I ran the S1 cab solo from that same Eden head, which was probably only putting out MAYBE 400W max, and had no trouble keeping up with vox, keys, and a pretty loud drummer. Very impressive as a standalone cab especially noting its size.
IMHO, the UL410 still reigns king as far as the loudest punchiest, and most "worthy" single cab solution out there, but it's modest bump in dimensions make a more than modest bump in its "carry-ability" for me, so it's not really an option. The 310s are amazingly easy to carry.
Regardless, if you get one or two, definitely get the snap-in caster set from Epifani. Very cool, and even come with the rubber feet to protect the caster baseplates.
Hope this helps.