We are in a 2006 Marathon XLII, and love it. There are enthusiasts for both models, though, and in order to truly understand the difference, you need to look inside both yourself. Picture yourself living in it, and consider what is most important to you. The XL's have more inside storage, and rumor has it that the H's have more outside storage. The one thing I'm sure of, and not just because I'm an XL owner, is that there is a bit more living space in the XL because the passenger seat turns around and joins the living room. In the H, because the passenger seat is on a lower level with the driver, the living space is a bit shorter.
Having said that, consider your co-pilot as well. I know I can put up my footrest, but I swear that at least in a Marathon, the co-pilot's comfort was considered as much as the pilot! I'm 5'4", and my feet dangle unless I put up my footrest.