The LP85 and the HP100 are almost identical in size. I own and dive both.Al40's should only be bailout def not enough air no weight. I think if you are having a situation where you need to constantly remove tanks to get through a tight restriction i think Al80's are the best cause they stay neutral for a little bit. I personally Dive AL80 and LP 85. I hated diving hp 100 cause they are short and a pain to deal with the 85 has the length and small diameter so for it me it fits my profile better.