27 is nothing, that you just about dont feel. 25 is very nice, slides right in, the 27g is short and really not terribly convenient in hard to reach spots because you do need some good pressure to push through it. Terumo has a stronger base than BD so I do better with them.
Yes there is a difference with 23/25, just as with 25/27, but 23 is ok. I just like as little trauma as possible, especially since my body loves to bleed. When doing ED injects also, it gets pretty bad with scar tissue after awhile.