How to evaluate different alloy types used for bicycle frames?
For all I know, between those two aluminium alloys there's no rule of thumb that could say: "this one makes stronger frame" (or lighter with the same strength, whichever way you look at it).

From a mechanical point of view 7005 is stronger, but also more brittle - and that needs to be taken into account when designing tubes and the whole frame. 

So you can build relatively light and strong frames from both these alloys, without one allowing an ultimately supperior frame compared to the other (in terms of better strength, durability, lower mass etc).

The material of choice does boil down to price - in terms of what equipment a factory has at their disposal to machine the frames, since the two materials require different processes for heat treatment, aging etc, cost of material (alloy) and so on. Then it's up to the engineer to design the frame with the chosen material.

Design challenges are making frames as light with enough strength and resistance to impacts. Butted tubes, making them stronger where it's needed and lighter where the strength is not cruicial, distributing the loads along the tube length, away from critical areas is a way to do that. Exactly how much material and where has been perfected in the previous decades, didn't see much breakthrough lately, probably won't in the future as well. It's gotten as good as it gets, just copy/paste the design, and let the poorly paid Chinese people build it for you (Chinese factory equipment could dictate the alloy of choice, whether they work with 6061, or 7005).

Whichever material was chosen, the rest is up to marketing - to say that 7005 is stronger, or that 6061 is more elastic, or whatever. 

What makes the difference is the quality of frame design and how well that is built (executed). The more time and money you spend on testing (with good engineers), the lighter can a frame be, still having enough strength (and durability). Though, like I've said, I see no breakthroughs in the past decade in these terms.

As a buyer, with marketing being put in front of engineering for the past decades, it's really hard to say which is better when choosing a frame, regardless of the material. On the plus side, metal frame designs (especially steel and aluminium ones) have been tried, tested and perfected, so you can hardly go wrong, especially if choosing a renowned manufacturer. Having said that, the cheaper, "no-name" frames, while being much heavier than needed, are often even stronger and more durable, they just "weigh a ton".

For welding aluminium frame - I don't think that you can expect to weld-repair an alu frame to be durable, without aging or heat treatment afterwards, whichever alloy was chosen, or at least adding a lot of extra weight. 7005 requres no heat treatment after welding, but I don't think that the cost of a durable weld repair is as good as it is for steel. Someone with more knowledge on repairing alu frams could probably answer that better.