I decided to try this out for myself, though I don't own Urban Decay Primer Potion, because that is usually what people prefer.
The price: The TFSI primer is a whopping £16.50 ($18) which isn't really up my street. I got mine in a set from TKMaxx, alongside 4 other products, for £12.99 (incredible, I know).
The ELF primer is only £3.00 ($3.00) which is just perfect - it won't break any one's bank and therefore can be used more liberally.
The consistency: TFSI, I found, was thicker and creamier than EMEP. I personally don't like that, and prefer the sheer-er application of the ELF primer, but that's just me. Usually a thinner coat results in poorer performance but I don't find that.
The smell: an unusual criterion for an eyeshadow primer, but I found that TFSI smells really bad. It is very unpleasant, so much that I don't enjoy using it because my fingers smell like dried up chemicals and I don't like the idea of that being on my eye. The EMEP smells faintly of chemicals, but a much more pleasant smell and much weaker, which I prefer.
Application: the TFSI comes in a squeezy tube with a thin nozzle, whereas EMEP has a wand applicator. I much prefer the ELF applicator because I get less on my hands (even though I still blend with my fingers) and waste less product. I always manage to squeeze too much of TFSI out and it goes everywhere and I have trouble dealing with the excess.
The performance: the most important comparison! I found that I actually prefer the ELF primer. It wasn't 'better' as such, but all the pro's of application, consistency and smell have definitely overthrown TFSI for me. The eyeshadow stays put for the whole day, and I actually noticed that ELF lasts a little longer, as TFSI starts to crease after about 6 hours (only in the corners, slightly).
Verdict: I'd choose ELF anyday. It is perfect - I can't find a flaw in this product and at £3.00 it is more than worth it. I understand expensive makeup when the price matches the quality, but with the Too Faced Shadow Insurance I found that it is just an overpriced product with a performance similar to a much cheaper product which has more benefits.