Have you used downtube shifters while riding a loaded touring bike? It's been many years, but I don't recall it being all that much fun (and this was back when all I rode was bikes with downtube shifters and it was second nature to me). For flat bars I'd sure lean to shifters on the bars. I know you can't really go friction with most (all?) flat-bar shifters, but they are built so robustly that it's fairly rare for them to break (compared with STI drop-bar shifters, for example). And they are pretty much the standard world-wide, so a replacement should be easy to find (not so much if your downtube shifters break).