I've owned all of the following, so take my opinion with a grain of salt: 2011 UL, 1993 Classic, D120, D60, Sig:X. I listed the years for the first two because I know there have been some revisions.
In order here's how I'd rank them for my purposes:
1) Sig:X
2) Classic
3) UL
4) D120
5) D60
Since you asked just about the Sig:X as compared to the UL and Deliverance...
The Sig:X is the most versatile and has the best clean channel. Rhythm and Lead channels are still very good--this amp's distortion channels fall somewhere right in the middle of the CLX/Classic, UL, and D60. You can really sing with this amp and it's not as dry as the UL, but it doesn't cut as hard as the CLX/Classic. The Clean channel makes this my favorite Fryette amp though. You can really use it like a non master volume amp, and I could see recording with just this channel for a lot of stuff. This channel is what gave me the warm and fuzzies and made it clear that I wasn't walking out of the store without that amp. The only other two amps that have ever made me do that were the Bogner 101B (which I just couldn't afford anyways, so meh) and the Pittbull Classic I still use. and eventually plan to build a head cab for and use as a head on my Fatbottom.
The Deliverance is one of the best single channel amps I've ever used. It's biggest competitor in that regard is D13 which I think I actually preferred, but they're different. The D120 and D60 definitely have different characters. I preferred the D60 for single coils and/or more rock and hard rock and D120 for humbuckers and more aggressive sounds. Either amp can get clean but it's not a "true" clean to me. There's still a little grit, which I'm okay with. Single coils on the D60 really got me into Blood Sugar RHCP and Hendrix land. I could get VHII type sounds out of either amp with a humbucker. To me, the D120 is everything the Peavey 5150/6505 wants to be.
The UL, frankly, was my least favorite of the VHT's. Great amp, but I guess there's a reason you'll rarely see it outside of a heavy music context. The cleans were almost too hi-fi, and the distortion didn't bloom like the other amps. This amp is GREAT for metal and I can definitely understand why Page Hamilton would use this amp. It just wasn't my cup of tea, which isn't to say I didn't like it (I liked it a lot!) but is to say that, among the Fryette amps, this one is the only one that didn't put a huge smile on my face. I don't have a heavy picking hand though I can get there when necessary. That said, I never got to use it with a Fatbottom cab, and I hear that can be a big difference. The reason I ranked it ahead of the Deliverances above is because channel switching is necessary for me.