i'd say come up with your proposed design before you trash the idea of using the 1/8" wall tubing - especially if the alternative is 1 1/2" solid square. lighter weight with a little reinforcing may well be as strong as or stronger than the solid alternative, and at a lot of weight savings. I don't know what you have for a tractor, but mine is very small - something like 850lbs lifting capacity in the bucket. because of that, when i make any attachments or modifications i want to keep the weight at a minimum, otherwise i might not be able to lift anything other than an empty bucket.
when you think of strength, section depth is generally king, and not area. think of floor stringers: a 2x8 and a 4x4 have the same theoretical gross area, but if you were using one of them for a stringer you would choose the 2x8. steel is really the same thing. the solid 1 1/2 will be better suited to avoiding damage from crumpling, but it would probably bend similar to a 2x2x1/4, which is significantly lighter. on a loader attachment frame you're talking about something roughly 4 feet wide and 16" high. that's a fairly stout frame, and depending on how you brace it it could be fine.
the biggest issue i see when using thin steel is fatigue around bolt and pin holes. in those cases i'll generally weld a thicker piece on where i have a hole in order to strengthen the section and give the bolt or pin more bearing surface. otherwise, i'll make the hole even larger and weld a sleeve in for the bolt or pin to pass through.
