I'm fixing up an early-to-mid '80s Bianchi ten-speed I found at a garage sale -- my first bike since the MTB I rode as a kid. I plan to mostly use it for errands around town, plus the occasional recreational ride.

I'd like some advice about shifters. The Bianchi came with Shimano Z401 stem shifters (which just clamp onto the stem, so they'd be easy to remove). I also have a set of Suntour downtube shifters from a project bike that didn't work out; the Bianchi has a cable stop at about the right place on the downtube to hold a clamp in place. I'm trying to do this on a shoestring budget, so STI's aren't an option, but bar end shifters from eBay are.

What are the pros and cons of stem, downtube, and bar end shifters, for mostly-urban riding?

For what you describe, the bar ends would be the best for you by far. I have bikes with all three types of shifters. The stem shifters are awkward to use in general but they and the downtube shifters require one hand off of the handlebars when shifting. This can be a problem in traffic. The advantage to the stem shifters is if you want to ride more upright all the time, not in the drops or on the hoods. The downtube shifters are fairly convenient if you are in the drops most of the time. The barend shifters allow you to keep your hands on the handlebars, give you precise shifting and are not as awkward as the stem shifters. Hope this helps.

You'll probably hear a lot of arguments for and against each of those for urban use, but it's my opinion that you'll get used to whatever you have. I use both barend and downtubes and never have had a problem quickly & safely reaching either. I'm not so sure about the stem shifters though. It seems a little risky to have pointy things in that general area

In case you did not see it, a lot of your questions were just addressed in this thread:

"Putting drop bars on hybrid, cheap commuter"

it's about half way down on the current page. Personally, I like the stem shifters as they are very easy for the eye to find (you don't see people mounting their computers on bar ends or downtubes, now do you?) without having to take your eyes off the road. With drop bars I am riding with my hands up top most of the time and the move to the shifter is very short. This is, of course, personal preference. You will get plenty of arguments in favor of bar end shifters and down tube, as well.

Before ditching the stem mounts,check for cable guides and stops in the right place on the frame. Many stem mounts used continious casing and present prolems when switchng to downtube or barends.

I am not familiar with those specific stem mount shifters, but the stem or headtube mounted shifters I have seen are simply downtube shifters on a different mount. If that is the case you could just take the shifters on and mount them on the downtube shifter bosses. How hilly is the area where you usually ride? How often do you actually shift? I have used both STI and barends. Both are great, but I have gone back to downtube on my commuter for simplicity. But then I seldom need to shift. Except on the rare occasion when you when need to shift unexpectedly it really isn't that big a deal to use downtube. As for bar ends, unless you ride in the drops all the time you will probably have to take you hand off the bar momentarily.

Well, most of them do not work that way. Very few if any use the type mount that works with a downtube boss,and his frame has no bosses anyway. The stem clamp is too small in diameter to be used on a downtube.

While this is true, the difference here is that you have your hand on the handlebars while shifting with barends. You can choose the time to move your hand there. With other shifters, when you need to shift you have to have your hand off of the handlebars. In city traffic this can be a problem or at minimum a pain to manage.

I have those great old SunTour ratcheting barcons on the UO-8, plus a spare set I may put on the other Peugeot. They are superb, provided they can take up enough cable for your application. The mountain bike, with its SunTour Accushift thumb levers in friction mode, is very easy to ride confidently in traffic.

However, I do plan to keep the original Campag. downtube levers on the Capo and the Bianchi.

I think he was referring to just the levers, not the bosses and clamp. Even so, it isn't true across the board.

I posted this somewhere else, but it applies here to. Another alternative is Suntour Command shifters. You can probably find a set on ebay if you look long enough. They mounted on the handlebars inboard of the brake levers and were designed so that you could shift from virtually any position on the bars. They indexed to seven speeds or switched to friction mode. Kinda funky looking, but worked great.

There are some pretty good hills around. I figure that the more convenient shifting is, the more I'll shift (on my MTB with thumb shifters, I used to shift a whole lot; thus far, I've hardly used the stem shifters at all), so the more efficiently I'll ride.

That does sound handy. I went ahead and bid on a set of bar end shifters on eBay, but if I get outbid, I'll keep an eye out for a set of Suntour Commands.

I know what he was refering to, and turst me the majority DON'T work that way. I have a whole box full of different kinds.

I don't know how up to date the guy's inventory list is, but he has a set for sale for some ridiculously low price (I'm thinking it may be a misprint). I have bought stuff from him in the past. He's legit.

https://home.tir.com/~lbicosab/pricelist.htm