If you had time for only 1, which would you choose?

Can't really answer w/o knowing more -- like what time of year, whether you want to hike or not, things like that . . .

You will get different answers from everyone. Both are fantastic, but Bryce just blew me away. I was in awe of the beauty.

More hiking, more variety in Zion. Bryce is beautiful but can be done in a day. Zion has some of the most unique hikes in America.

I liked Zion better. Bryce was ok but I only do small hikes. I was there in April so there was some snow in Bryce. You could definitely stay in Zion with a day trip to Bryce if you are planning on staying for a couple of days.

If you've given yourself even two days for the visit to this area, then one day in each will work fine. If more than two days, then give Zion the edge.

If you've given yourself one day to the area, kick yourself for that decision and then decide whether you want to do some significant hiking or not. If you do, choose Zion; if not, choose Bryce and resolve to come back and give yourself enough time to do both right.

BTW, if you're visiting Las Vegas and gave yourself ONE DAY to get to this area, I can assure you that NOTHING in Vegas is worth missing either of these scenic wonders. I'd cut back on my time with the fake beauty in Vegas and see something of real beauty in these parks.

edjim is correct

Vaga

I much prefer Zion over Bryce - far more to do. But in terms of uniqueness, as much as Zion is beautiful & amazing, the expanse of hoodoos at Bryce is other-worldly.

It's like asking which child do you like better?

Both are stunning. I think Bryce is the absolute, single most beautiful place I have ever seen. Zion, however, has more "fun" things to do and deserves more time in my opinion.

If you give us more details about when the trip is, where you are coming from or going to afterward, and how much time you have, you may get more helpful results.