Best way I could put it to describe their differences:
100W Pittbulls punch you in the face/head until you can't take it anymore.
Deliverance 120 punches you in the liver and diaphragm until you're on the ground trying to catch your breath.
As for power amps, I remember reading somewhere from someone who would know (might have been Fryette Support, Steve Fryette, or Stephen Sawall who is a Fryette encyclopedia himself) that these are the basic VHT/Fryette power amps:
Ultra Lead and D120 effectively have the same power amp section. So if you run a GP3 into the return of a D120 (with a loop), then you're effectively playing a Pittbull Ultra Lead with all of the other bells and whistles that the GP3 comes with (midi, independent EQ on each channel, etc).
Sig:X with diode rectification is one side of 2:90:2 power amp.
The 50CL power amp is the same as one side of the 2:50:2 power amp.
The CLX/Classic/100CL power amp is the same as one side of the 2100 power amp.
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Overall advice I'd give is that if you're truly torn between both (and cannot buy both because reality sucks), err towards buying the D120.
It's the amp you haven't tried yet and you'll keep wondering about it until you do. Furthermore, I'm assuming it's a reasonable deal on it... if you don't like it, you should be able to flip it pretty easily.
The 100CL is regularly compared unfavorably to the flagship Pittbulls (CLX or UL), and its used value is usually lower and they are less sought after. The Deliverance series' used value seems to have jumped quite a bit in the past few years or so with no sign of it coming back down.
Additional food for thought: I don't think I've ever seen someone say that the 100CL is the best amp they've ever played... desert island yada yada. I've seen a few people say such things about the D60 and D120.
I still have a Sig:X and a CLX with EQ. I love both amps, and both can approximate what the D120 and D60 used to do, but they can't nail it. I do miss my Deliverance amps but am biased by the deals I got for them at the time. In 2012 I got a second-hand D60 for $600. Then in 2013 or 2014, I got a D120 (with an insert loop installed by Victoria Amps) AND a FB412 straight cab with P50Es for $1000 all in. Seeing their used values jump into the $1500+ range for just the head kind of makes me say "no" just because I know where their values were relatively recently.
There are only 3 amps for which I really have GAS... mainly because they're amps I owned and remember very fondly: Mesa Trem-O-Verb, VHT D60 or 120, and Peavey VTM-60.