Each one has its own pros and cons. Tree peonies grow taller, make bigger flowers, but they also grow slower and flower for less time.

Itoh peonies can flower for several weeks, they are easier to propagate, and produce more shoots. They are more prolific as 1 little shoot can produce up to 5-7 shoots the following year while tree peonies sometimes only put up 1 or 2 every so often. Their weakness is smaller flowers, less varieties and their woody sections don't grow as tall as tree peonies.

I have plenty of tree peonies however I got a decent deal on eBay from MACAO2000 last spring where I acquired 5 species of itoh peonies. Here's my blog post on them. I believe that you should have both.

Here is a link that might be useful: Getting 5 of my itoh peonies