The first difference I noticed is that Urban Decay's Primer Potion has more ingredients. The weight of Too Faced Shadow Insurance is 11 g/ 0.35 Oz, and Urban Decay's weight is 10ml e 0.34 US fl. oz. The formula for Too Faced Shadow Insurance is a lot runnier than Urban Decay's (and you have to shake up Too Faced Shadow Insurance). Urban Decay's Primer Potion is almost a mousse.
10:40 AM, I've applied both Primers on my arm, let them set for 10 minutes, then applied Make Up Forever's Neon Pink eyeshadow.
10:40 AM, Urban Decay Primer Potion VS Too Faced Shadow Insurance
12:40 PM, 2 hours later, both shadows are still looking well pigmented and silky. The only difference I can tell so far is that Urban Decay's Primer Potion is looking a little chalky near the bottom of the swatch.
12:30 PM, Urban Decay Primer Potion VS Too Faced Shadow Insurance
2:40 PM, 4 hours after application, at this point both of the shadows are still looking very similar. The only main difference that I can see is that Urban Decay's Primer Potion chalked a bit near the bottom of the swatch, and the swatch with Too Faced Shadow Insurance seems to have a little more colour intensity.
2:40 PM, Urban Decay Primer Potion VS Too Faced Shadow Insurance
All in all, I am very undecided about choosing between these two primers. I'll definitely be doing a longer more in-depth review and experiment soon.