The main difference is that the Sleek version is under a fiver for 5.4g of product, and Illamasqua's is around £20 for 1.5g of product. Surely this means that Illamasqua should be far better?
Sleek offers 2 colours in a nice little mirrored compact, and Illam (as it shall be known for the rest of this post!) has the one colour (mine is Enrapture) in a small pot. In terms of pigmentation, as you can see from the below swatch Sleek is amazingly pigmented and does indeed earn the title Molten metal. Illam does have a lovely pigment but did need a few layers and still apeared patchy. I think it looks too dry to look like a liquid metal- it almost looks powdery.
The textures of the 2 are also quite different. Sleek is very smooth and creamy and Illam is thicker and dryer. Illam does feel of better quality (and smells lovely). Because Sleek is creamier it isn't as long lasting and tends to have a slight moveability.
Overall, i do love Illamasqua's Liquid Metal. When they first brought these out i had never seen anything like it, but saying that the Sleek product has to win for me. The value is amazing (Illam in my opinion is by far overpriced) and the product itself is very good. I would like to see Sleek release a selection of different colours (which i'm sure they will).