I've only ever been to each one once. I went to Pizza Hut a few weeks ago for the first time ever and found it surprisingly nice - good value for money with all you can eat buffet.
I went to Pizza Express earlier this year for a college evening out, and am going back there with a group of friends the night A-Level results for a celebration OR commiseration meal.
On the whole, I would class Pizza Express as being slightly more 'posh' than Pizza Hut, it is also quite expensive (well I thought it was). However Pizza Hut is great value for money and is good if you're just going out with friends for lunch/a bite to eat.
In the end, both have their upsides.