I agree a motivated rider can get fit on any bike .Many bent riders mention it relieves stress on joints and that may be a factor to you in putting in the miles to build fitness.On the other hand try the comfort for an extended ride and if your joints don't complain you may be able to save some money(keep in mind its usually well into a sustained ride that the joints start acting up).As well some limited stress on the joints(if not enough to keep you from riding)may be beneficial in strenghtening connective tissue,supporting muscles etc(sort of the use it or lose it philosophy).In such case proper bike fit and range of adjustability of the bike and your positioning on it become important factors. Good luck and get out riding whichever you decide on.