I can't address the 'rust' issue.....I own 2 Faber LP 120 steel's, but use protective tank mesh, so no rust issues as of now.
I've been very pleased with mine so far.....LP's can be 'overfilled'........so if you only go up to 3300 PSI, you get 150 cu. ft. of air......which is like diving a pair of AL 80's, but much lighter and less bulky than doubles.
Just make sure you can handle the weight.......you might want to borrow/rent one to see if you can cope with it.
I'm not a big guy, and at 1st they were a little intimidating, at least on dry land, but it wasn't long before I got accustomed to it, so now I'm comfortable with them. and the added safety/endurance is well worth it.
None of my local shops rented them (the biggest anyone offers is an AL 100) so I took a chance on them and bought them on faith......which worked out well for me.