To me, the quality of a rim is determined by it's design testing and rating, not by its metal of construction. A steel rim may be superior to an aluminum rim, or the reverse.
One possibility to increase the load capacity of your tires in a 15 inch rim size is to go to Maxxis Tires
. Maxxis (and I think Goodyear Marathon) have a load range E trailer tire. For a while Maxxis was almost universally acclaimed, and then got a smattering of complaints. I've got in excess of 25000 miles on a set and so far have had no issues with then.
Even if you keep tires in a 15 inch size, you might have to replace your rims due to the increased air pressure that a load range E tire requires. Your current rims may have a paper label them stating their pressure rating, or the rating may be stamped on the rim. Obviously you don't want an 80 PSI tire on a 65 PSI rim.
There's lots of advice and information in forums... sometimes it is correct. For example, all of my posts are made by a political appointee who got the job as a reward for contributions to my diesel bill.
2011 Jayco 28.5RLS; 2008 Chevy Duramax; Pullrite Superglide Hitch