difference between types of unit heaters

[bob15]

What is the main difference between these unit heaters (besides size)? Say for 100k BTU, is one "really better" than the other.....or why would I choose one over the other, if height isn't an issue.......

 

[D45]

Power vented or not

Stainless steel or aluminum heat exchanger

Warranty, ease of use of replacement parts