I had a look at both the iCom and StandardHorizon, and ended up getting the HX851E at a good price from Delcom.
IMHO the functionality of the two radios are sufficiently similar that it just comes down to personal preference. If one had volume/squelch knobs then I'd have gone for that over the buttons.
I'd encourage you to go to a store and fondle each of them to see which feels better in your hand. They are both quite chunky, and can be a bit of a fiddle to work one-handed while maintaining a good grip.
I've been happy with the quality of the HX851E, sound quality is very good, menus etc are the usual multi-level choices to get to anything, but easy enough to select channels and adjust vol/squelch which is what I spend most of the time doing. Battery life is very good, time to GPS fix very good, and as Peter M wrote, you could use it to navigate to waypoints, but that would only be if you are desperate.
The HX851E belt clip is not super strong, I guess it is a compromise between holding it on your lifejacket securely and making it easy enough to remove and use. I think I'll investigate ways attach it via lanyard.
I then went on to get the VC-27 earpiece, which is crap. The wire is *very* thin and the PTT button stuck on when wet. I've since replaced it with a (third-party) D-ring earpiece and boom mike which is much better.
Can't help with info on living with the iCom as I don't have both.