So is it better to go from a smaller pipe to a bigger pipe?
I don't think the shutoff valves leaking should have anything to do with the diameter of pipe you're using. It sounds like the situation is: you are replacing that last bit of pipe anyway because the shutoff valves are leaking, and you want to know if you can also increase the pipe diameter while doing this.

Changing the pipe diameter will not do anything to fix leaky shutoff valves, and it probably won't have any benefit, but I also don't think it will do any harm.

If it is less than a foot of travel, it's hard to see how flow could be impacted  much, unless it is getting constricted from something huge down to a half-inch. Keep in mind that the flow will be restricted by the smallest diameter pipe. If a 1/2" line is what's coming out of the wall and you couple it up to 3/4" for the last foot, you're not really changing much.