Do this. It will line up better with your budget. If I were shopping the co-op and found a frame that fits me, in nice condition, I would not care much what it's made of, and even less about its tubing profiles. Any variety of chrome-moly is more desirable than high tensile steel. If it is chrome-moly (from any brand) it will have a sticker to say so... unless, of course, it has been stripped or repainted. Plenty of fine frames were made of aluminum in the 90s and 2000s, and the alloy (6061, 7005, etc.) isn't very important, considering your budget. Focus on condition, size, and whether it's configured for the kind of components you want to use (brakes, headset, bottom bracket, seat post, etc.)