When I first started IVing, I used to always use the smallest needle possible, 31g, because it seemed to make the most sense to have it as small as possible to leave the smallest mark possible. Also I didn't reuse them, so I didn't care that they dull fast.
Then I came across free needles, which were 28 and 29 gauge, and used those. I didn't like that they were bigger, thus potentially causing the largest mark, but they worked fine, and as others have noticed, its easier to register. Actually, I'll admit, however, I never register. Assuming my needle is sharp, I can feel it enter the vein so I never bother registering. After pressing 1 unit in I can tell if I'm in or not so it doesn't bother me, its just my style; when I register I fuck up and pull it out of the vein.
Also, these larger gauges don't clog like 31s do. I swear with 31s it seems like the needle clogs every 3rd or 4th time. With something like 28s or 29s its way, way less.
So from my experience of using both, I'd say not that I prefer 29 or 30 gauge. Still small enough not to leave a large mark, but not so small that they clog often, and also not too hard to register with, if I do that. I would register if I was doing a shot for someone else, because I can't go by feel then like I can when hitting myself up.
(I'd like to note that I haven't shot up for something like a year, but I wrote that in the present tense because its easier to write that way.)