Much easier to stretch a slightly-too-small tube than to try and stuff a somewhat-too-big tube in a skinny tyre.
Neither of your cases are outside the realms of "easily doable" though.
With a radically stretched tube, the danger is that if you get a flat, they sometimes pop like a balloon and rip a big hole in themselves. I've had this when using a 26x1" (MTB-road) tube in a 26x2.3" (MTB-offroad) tyre. Worked fine for ages until I got a flat, but wasn't repairable when it went bang. I'd completely forgotten it was in there (must have been the only spare tube I had at the time) until I pulled it out to patch it and saw how skinny the intact bits of it were.
On the other hand, if a tube is bigger than the inside of the tyre case in its natural unstretched state, then it will fold and bunch up inside the tyre. That can be bumpy to ride on, and can cause the folded bits to fail due to abrasion and also due to being bent so sharply. But a 35 tube in a 34 tyre isn't likely to cause this problem if you install it carefully.
I'd go the 25-28s.