I prefer Upper Pines for the valley access reason you mentioned. Wawona is very nice, and had virtually no snow when I was there last week. It's a whole different environment, but the valley waterfalls should be pretty impressive by then, so I'd (and we are) pick the valley.
March weather is extremely unreliable. It can be sunny and warm, rain, snow, or be cold dark and foggy, or all of them within a few days. Just be prepared for all of it...bring warm stuff, shorts, and perhaps a tarp or two to stretch over tents if it rains much.
Through March you should be able to have fires during the day, but if you can do without, please do. The valley gets very smoky, and if you keep busy you really might not need a fire like some folks seem to...burning and smoking all day and evening. I bring a few presto logs and an armful of wood, but often never use it. If anything, just in the evening before retiring. They sell wood at the Village, about $9 a box but if you use it sparingly, there's quite a bit.
Besides hiking around the valley loop trails, you can go beyond Mirror Lake on about a six mile loop, which is a nice non-strenuous but scenic hike in the shadow of Half Dome and next to the Tenaya River. You can get to Vernal and Nevada Falls via a combination of the John Muir and Mist trails...both of them have parts closed in winter, but you can go from one to the other above Vernal Falls. You won't be able to go up Half Dome, but there's no reason you couldn't do the hike up TO the base of the cables (long and strenuous, regardless).
The easier trails to lower Yosemite Falls and Bridalveil falls are a must see, though they're very busy. There's an upper Yosemite Falls trail, you might set your goal to as high as Columbia Rock, which is about a mile, but seemingly straight up (well, zigzag straight up 8^). You may get up beyond that to the base of upper falls, really worth seeing. Above that is possible that it will still be snowy and icy, it's hard to predict.
You can drive up to Badger Pass for snow fun or scenery (bring chains) and though Mariposa Grove is way down at the South entrance, it's a nice walk with some giant redwoods as a reward (2 miles uphill along a possibly snowy road). If you're there around the full moon (21st) they may have some nighttime ranger snowshoe walks.
Those, with a couple of days spent just doing the valley loops between the campground and El Capitan, should keep you very busy. Don't expect too much 'green' or wildflowers, it's a bit early for that, but hopefully there will be some sunny weather and things look great regardless.
If you have bikes, most definitely bring them. You may be able to rent them in the valley, but having one at the campground is great, and frees you from the shuttle bus wait.
There's food and showers at Curry Village nearby, and food at the Lodge and Village, should you not want to cook a lot. Enjoy your visit!
Post Edited (03-04-08 22:42)
Gary
Yosemite Photo Galleries: http://www.pbase.com/roberthouse/yo