I liked the SW course better than Avengers. Like what's already been mentioned, you go through Garden Grove which we've never been done before and I really enjoyed it. Possible cons to that course could be, depending on how you like your courses, there is a long section on Harbor and you run up it and back down. I didn't mind it, but some people don't and if you're in the back of the corrals you start to see people coming back up when you still have awhile before you get back there.
Also depending on the weather, there's a section where you're running back to Harbor and if it's sunny, you're running directly into the sun which could be a pain. I didn't dislike Avengers, I just liked SW better. You get the stadium which is fun and go over by the Crystal Cathedral which is a So Cal icon and was nice to see up close. Like Kirstie mentioned, there is a long section on the trail, I didn't mind it but some people do. It was nice b/c the 1st mile was were the cos players were at and the second you could finally see the stadium and it gave you something to run to; it was broken up really well.
I think it really comes down to which themed race you want to run and like Jess mentioned, what you want to see in the parks. SW is on MLK weekend so you might have that built in day off of work on Mon, but there are crowds in the parks. Sun has been really busy the past 4 years. HM may or may not be down, b/c they're taking down the NBC decorations (down this year, but regular HM opened during that weekend in the past) and IASW mostly likely will still be Xmas, but everything else is regular DL decorations. If you run SW in '16, you might still be able to catch some of the 60th decorations; the 50th stuff lasted into the following year a little last time. Avengers was the 1st weekend of Xmas and really busy. I think that weekend was more crowded than SW, but you get to see all the Xmas stuff.
Weather and time of the year doesn't really matter ~ I was in capris and a short sleeve both weekends and shorts during the day. There was 1 Tink a few years back(now the SW weekend) where is was colder for the 5k, but fine during the day. Again like Jess mentioned, you could have Santa Ana's in Nov and colder weather in Jan but honestly, it's CA and you really never know until sometimes the week of if there's going to be weather. But that being said, that's another thing to keep in mind is where you live and travel. Depending on where you live, if you have a lot of snow, it might be easier to come out for Avengers. I've done WDW in Jan a few time and W&D last year, and coming out from CA I've always had to look in Jan where my layovers would be so I wouldn't get stuck or cancelled while W&D was awesome, b/c I knew it wouldn't be a problem if my layover was in a snowy place.