The speeds are about the same on I-70 and I-80, although it is only 65 mph in the Colorado mountains. But I-70 goes through the mountains and I-80 goes around them. Wind and drifting snow on I-80 vs. snow-packed/icy conditions and high mountain passes with steep (6%) grades on I-70 at Vail Pass and the Eisenhower Tunnel. Of course, we can also have 50 degree shirt-sleeve weather in January!
So, the choice is really up to you. But with only two-wheel drive, I would take I-80--shorter, less ice, lower grades, and plenty of motels in every little town from Elko to Green River to Cheyenne, in case a blizzard should strike. If you use I-70, you may be required to put on chains (trucks and trailers only, generally not cars). That may be no problem if you also have to do it in the California mountains, but a real pain if you don't.
Oh, and don't forget to come back in June for the 54th Annual Rallye Glenwood Springs put on by the MG Car Club, Rocky Mountain Centre. Then you can drive your LBC with the top down through Glenwood Canyon and Aspen and really enjoy what Colorado has to offer! Go to "www.mgcc.org/rmc/htm" for information.