I have 2 of the Shorai batteries in my bikes. One small one (I think the 7 or 9A/Hr model) in my kick-only XS650 and the biggest meanest one they make for my BMW R75 (I actually won that one, didn't shell out the $350 they cost). Like them both and have had no problems with either.
I don't know much about the Antigravity, other than many of the new lithium motorcycle batteries use basically the same cells. Shorai describes theirs as being different.
The Antigravity unit is very small compared to the Shorai, particularly thin. So if you needed to slide it in flat somewhere, it's a good choice.
Either would be fine. If you went with Shorai, I'd consider spending the extra $15 and jump to the 9A/Hr version which is exactly the same size (case #2) as the 7. Shorai makes all the way up to a 14A/Hr 210cca model in that same case. I think they figure that's as small as anyone really needs so why bother with the expense of a smaller case for the lower capacity models.