C&V habitue Doohickie's post brings up, indirectly as with the rest of ours, that there's a big subjective aspect to this. Only your experience will tell for sure. But another point to consider is: with a bare minimum of work it's easy and cheap to switch from one to the other. Or, less cheaply, to bar-end shifters if you want to try those. (Or, expensively, to modern brifters) You can get clamp-on downtube shifters to replace stem shifters, and vice-versa, on eBay. Requires only a little work and maybe new cables (if you go from downtube to stem or barends).
I only ask that, if you get a bike with downtube shifters on braze-on frame bosses, please oh please don't hack off the braze-ons if you switch to other shifters!