Bit late on this but never mind...
First singlespeed frame? Which one is better, the on-one has some funky sizing which worries me. I usually ride a medium/ 18 " frame with a 23 inch top tube. I think I would ride a 16" On one? Has anyone ridden both? Any reviews on the inbred?
I rode an Inbred for 2 years and am now riding a 1x1 - so hopefully can give you somehting useful...
I wouldn't say either is better than the other, but they are noticeably different. First off the Inbred is corrected for 100mm forks and the 1x1 for 80mm - so the Inbred has (in effect) slightly steeper angles for any given fork length. The 1x1 is a bit shorter and a touch heavier. I ran 420mm rigids on both and each had nice things and less nice things going on. The Inbred has a nice quick turn in, climbs wonderfully and the low SO makes it feel very agile. The 1x1 is more stable and has a reassuringly solid feel - I noticed the higher SO moving from the Inbred to the 1x1 but I'd forgotten about it in a week or two. The 1x1 is a more stable descending rough stuff (not so surprising with the slightly slacker head angle), but gives a slightly harsher ride compared to the relatively springy Inbred. The 1x1 is a bit more reassuring to thump off small drops etc (more solid feel and slacker angles).
Random detail - generally the finishing is a tad cleaner on the 1x1 than the Inbred.. but cable routing is nice on the Inbred (full length outers), tyre clearance seems better on the 1x1 (probably helped by the wider BB shell). Dropouts definitely neater on the Surly (compared to the standard horizontal drop Inbred).
Sizing - I'm 5' 11" and the 18" Inbred was spot on (16 inchers are too small) and the 18" Surly is spot on - but they feel
quite different sizes due to the SO height - the Inbred is 18" bb to collar compared to about 19.5" for the same measurement on the Surly. The TT length difference isn't very noticeable - I've used the same 80mm stem on both and it feels fine.
Over here (UK) the Surly is priced a bit higher than the Inbred, but I'd say they're both value for money either way around.