6061 and 7005 are both aluminum alloys. In a nutshell, a 6061 monotube frame, not heat treated after welding, is going to break. I say that from personal experience. Before buying a 6061-framed Optima, even at a rock-bottom price, I'd recommend giving a close look at the welds. Specifically, look for cracks just outside of the weld line, around all the stays; and if the frame doesn't have the vertical stiffener at the headtube area, around those welds, too. It helps to have someone sitting on the bike while examining it, because the cracks will be placed so that they widen under load. If it's got any cracks, you'll have to decide it the bike is worth rebuilding it when the frame alone will cost $500 or more.

I'm on my third Baron frame; and when I bought it used, the frame had already been replaced under warranty once. Optima has always denied frame breakage being a problem, but just in my circle of friends alone, there have been more frame replacements than what Optima acknowledges for their entire customer base. I expect the 7005 frame to last much longer; not because of the alloy but because it's (finally) heat-treated.