I'm leaning towards the former since monitors are cheap & I'm thinking the extra RAM will make up for the slower CPU (3821 vs. 6051 Passmark score), but I always have a hard time with this kind of trade-off.
Get the Vostro, no contest, get the extra 2gb of ram if you want. Changing/upgrading Ram is a cinch, processor you tend to be stuck with for longer, especially if it's not a homebuilt PC that is upgraded frequently.
If you get the vostro and it doesnt seem fast enough, you simply buy 2gb of ram and stick it in. In the other scenario what will you do, buy a new processor?
I don't see the i3 having trouble with MS apps. for years. True, the processor isn't cheap to upgrade, but the i3 3100 is still a fast processor. I hear 4gb is the target ram for a basic computer anyway.