The material appears to be mostly polyolefin however, the construction of all the different brands appear to be different.
Lavacore calls their's Trilaminate Polytherm while Hyperflex calls it polyolefin. Regardless of what it's called it appears to be a form of synthetic fleece that reduces heat lost by reducing water flow through the suit. Only the hyperflex 50/50 have a neoprene outer layer but I'm not sure about lavacore or fourth elements suits.
In terms of performance, it would appear that the fourth element comes first, lavacore 2nd and the hyperflex the third however, the differences appear to be minimal.
Personally I would choose the Hyperflex 50/50 cus of the 1.5mm neoprene outer layer, you could wear the top under the wetsuit and leave it on like a tee after the dive or use it for snorkeling or light water sports. You could add either a pair of polyolefin shorts or pants depending on how much warmth you need.
I'm only aware of lavacore producing full length pants.
Another thing, stay away from velcro unless you want to look like a fuzzy ball lol.
Cheers