I'm very confused, you listed SFO-LAX, and then you picked two airlines that don't fly that route?

The only reason I see to pick either of these for someone based out of SFO is if you don't care about domestic and will just be using someone like Southwest for that, or if you are counting on status-earning partners.

In that case, DL will merge with NW which will do nothig for you domestically (out of SFO), while CO is moving from Skyteam (its alliance with DL and NW) to Star Alliance (with UA, which has a focus city at SFO), so at least with CO you could fly partner flights on UA and then earn CO miles (including status miles) in a year or so.

Further confusing the issue: CO is only going to partner with UA, not merge with it, so CO in two years is likely relatively like CO today, while DL in two years will be half DL and half NW (and we dont' know which half for what). SO evaluating what DL is like today in the very specific terms you're asking about is not likely to mean much for more than the next year or so.

As to frequent flyer program on their own, both are among the most poorly-rated. CO's FFP is officially called OnePass, but most people familiar with it instead call it NonePass after the difficulty of redemption. But Delta with their new three-tier approach has made redemption at "saver" levels harder (than, say, on AA or UA) in the past year too. (But of course it depends a lot on your redemption specifics, there are some people who are able to redeem in any program because they're able to be as flexible as needed or whatever.)

But it all depends on what specifically you want from a frequent flyer program. Because the program that's best of upgrades may not at all be the program that's best for redemptions, for example. So even trying to rate a thing like "frequent flyer program" with one number can be very difficult.