Tio and Cathyl are very hard to choose between from for me, both are just absolutely amazing girls. So what it boils down to is minor differences.
I prefer Cathyls symmetrical horns to Tio singular forehead horn, but that's a very petty reason to pic one over the other. Cathyl has the HUGE bonus of naturally lactating all the time, yet I find Tio's aura of lovey-dovey-ness equally comparable. (Good lord the 2nd Tio date chapter was awesome) That said giving Cathyl the edge for her chest over Tio's personality is also super petty and a shitty thing to do. I would also point out that Cathyl's personality would soften as long as she knows she is the only one. However I can't make a decision based on head canon.
Cathyl having a cute tail does not make or break vs Tio not having one.
Hooves vs feet means nothing to me.
Tio does have the advantage of long feminine hair, but Cathyl's hair is tied up in a long braid or something, so she may have nice hair too, just not at Tio's level.
Farmers tan and physique vs naturally tan/dark with (realistically, no stick arms) some physical definition, but less than Cathyls. Not a tie breaker.
Cathyl may have fuzzy ears, but otherwise they both have similar ears.
Tio's redish eyes are cool, but Cathyl's green(?) and horizontal pupils are adorable.
And the minor tit-for-tat goes on with neither pulling a head.
And then there's Asterio :^)