A couple of comments, you seem to be mixing your highways and cities up a bit. CA120 runs out of Manteca -> Oakdale -> Yosemite. CA140 runs out of Merced -> Mariposa -> Yosemite. More importantly CA132 heads east out of Modesto ->Coulterville. I've driven (not ridden) all of these routes and would have these comments. 120 has some of the better shoulders and paving of roads in that area. I just drove it Friday. The only ringer is the climb at Mocassin to Groveland. Every highway leaving the valley seems to have one of these climbs but at least the grade, though long, is reasonable. 120 does have a fair amount of traffic.
140 is out of your way, pure and simple. It is a more exposed climb to Mariposa with little services. There is a great hostel though as the road drops down to the Merced River, Yosemite Bug. It would make a great overnight stop. The real drawback is that while you would have lower elevations to achieve to get to the park, you would then have to climb out of the park to at least 5000 ft. to make it to Camp Mather on the park road which does not have any shoulder - that is not attractive (to me at least). For this reason alone you would be better served to go 120. Not to mention that I doubt you could do this in 2 days on this route.
Your recently proposed route is an attractive option. While I've not traveled Keyes Rd., I have traveled CA 132 (Aug 2011) to Coulterville (which you will hook up on) and continued on to connect to 120 on J132. 132 has not seen much improvement since its construction but then it has little traffic. It is the classic undulating road to the Sierra. The kicker is that that nasty climb out of the valley here is much worse. The climb to Coulterville is steep (I'm going with 16%-20% grades) and exposed and the road that continues beyond has a similarly steep climbs in it. Again few services on this route.
For my money, I would take 120. And since you only have 2 days, 120 makes more sense.
Edit: I would think twice too about riding 140 up to Mariposa and then turning north on 49 to get to the Priest Grade climb. That just adds a whole lot more climbing (to 2300' on 140 or so) that you could avoid by sticking to 120 (gradual climb from Oakdale to China Camp to Mocassin), not to mention going the long way around.
