I like both long sleeves and armwarmers. In fact, in cooler weather I often use both. My dress to start rides starts with shorts and short sleeve jersey down to about 70. Then armwarmers that usually come off early in the ride. There's a small window where I will wear a long sleeve jersey; no warmers but I like it better when it's slightly cooler and I ride with a long sleeve under a short sleeve and warmers over the long sleeves.
I love warmers for the very quick and easy temperature control. Hit a hill, push them down. Starting down, pull them up.
And - off topic but quite relevant - newspaper if you are riding in mountains, not just hills. Stuffing newspaper down one's jersey is a probably 120 year old trick that the European pros still do. If you watch footage of a grand tour (and not the newsreel footage of the leaders) you will see the team workers who are well behind their team cars pull out a sheet to stuff their jerseys at mountain passes. In fact, I read in the Cycling News live coverage a few weeks ago of the prominent riders doing the same (it was either at the Vuelta or a monument). I often leave the house with a newspaper under my outermost jersey. Really easy to pull out, fold and stuff into a jersey pocket while riding. (Well folds take some quiet roads, but in a pinch you can always stuff.) And if it rains later - that newspaper could be a godsend. Newspaper also works really well on those long sleeve jersey only days,
And a comment on long sleeves - the really good ones for temperature control are the ones you can push up beyond your elbows without damaging the fabric. Again - the great hill temp moderators. (This often runs counter to sexy, trim, close fitting cuffs.)