Phoenix to San Diego
 

I have only about ten or so days to do a stretch, and I believe it's either going to be Phoenix to San Diego or Phoenix to El Paso, following the Southern Tier route

Any opinions regarding these treks? Looking at mid-March. Haven't been out to that part of the country, so any advice is appreciated. Wind, for example. I know must Southern Tier folks head east.

Basically, I need a stretch of road from one major city (airport) to another major city that is ridable in March, so the Southern Tier seems like the ticket. Plus, I've never been to the desert, so I would like to try something very different from the woodlands of the north.

Great Lakes,
Don't worry about the winds. They will be headwinds no matter which option you choose. Yes, the prevailing winds favor the west to east routing, but trust me, the local weather pattern that you encounter is the only thing that matters. Having just done the southern tour last year, west to east, beginning in mid March, your plan sounds good. Either direction is fine.
The route to San Diego will be mostly desert/high desert until you get to the mountains near San Diego. Lots of stuff to see and there will be other riders on the route as well.
The route to El Paso will put you into the mountains first and then flat lands to finish. If it is really cold/snowing in the mountains you can depart from the route at Safford, Ariz and head to Lordsburg, N.M. Or you can bail out of the mountains and go south to Deming as another option. You can use I-10 to get to Las Cruces for these options.
The Adventure Cycle maps are good assets for the trip. One map covers San Diego-Phoenix and a second map covers Phoenix-El Paso.
Good Luck and have a great ride.

Having just ridden from Oceanside to El Paso through Phoenix in October I would agree that the direction won't matter. You will have snow in the mountains though in New Mexico and the route through Globe might also be bad. The Lordsburg-Deming route is cool also - right through the desert. There are neat campgrounds on the border west of El Paso also (Pancho Villa State Park is good if its open - call). You can also camp next to the Apache Gold Casino in AZ if you want.

The other way taking 10 east from Phoenix to the border then south is a neat desert area. If you hit it on a weekend by Glamis (East of Salton Sea) there will be literally 10's of thousands of offroaders (or more!), so try to do it during the week. I can't really say which way is better, both were fun. Make sure to have thorn resistant tires and tubes.

Weather is going to be an issue no matter what. The thing is, if you are lucky you won''t have to worry about it, unlucky you will be in wet cold the whole trip. You will really have to watch.

Sounds nuts :)