I actually did the trails in opposite order (Wonderland 2013 and JMT 2014), and I did the JMT at a pretty comparable pace.
The one thing I'd note with the Wonderland is that, as noted above, it is very much a rollercoaster style trail. What this means from a practical standpoint is that the question when planning isn't so much "do I want to go an extra five miles today?" as it is "do I want to add on another 2000-foot climb today?". I don't think that there's a huge difference in difficulty between the two trails, but the difference in flexibility re: campsites does mean that these decisions will come up- on the JMT, you can basically just throw down wherever's usable/legal.
The Wonderland Trail was my first "long-ish" backpacking trip, and I wound up doing an 8-day itinerary (12ish miles a day) because I wasn't sure how I'd hold up hiking day after day. The trip was absolutely magnificent, but I will say that I finished an awful lot of days very, very early in the afternoons, even with late starts- I often started well after 8AM on a lot of days and often hit camp in the 2:00 range. (For perspective, I had enough time to get through 1.5 of the Song of Ice and Fire (Game of Thrones) books, which are massive.) The downtime actually was quite enjoyable and led to some of the more fun moments of the hike, but it definitely wasn't terribly taxing.
Finally- this isn't super related to the OP, but I found this planning tool super helpful:
http://www.wonderlandtrailguide.com/wonderland-trail-itinerary-planning/