If you catch it right and there's fresh snow on the ground, you'll love it. You could easily spend a week in Zion at any time of year and never get bored. Having said that, Bryce is close and I'd probably shoot for 3 days at Zion and 2 days at Bryce. Either way, you'll have a great time.
On another note, we'll be launching eFotoGuide: Zion National Park soon. This ebook is a comprehensive guide to photographing the park, including everything you need to know to photograph over 20 locations in and around Zion NP. More ebooks covering other parks and popular areas will be released over the coming months. If you're interested, here's a link to our website:
eFotoGuide