Deer Lake is probably a shorter drive, the parking is plentiful, and the lake is right next to the parking lot. The lake is a good size for a kayak and a great lake to learn. You're never far from shore and it's sheltered from the elements thanks to a bit of a dip in the landscape.
Pitt is a farther drive, plus an additional paddle upstream to the lake (which can be easier or harder, depending on tides). Pitt is massive and would be much harder to learn. It's also more open to the elements and is farther from aid if you run into trouble.
These lakes are SO different from each other, the answer depends very much on your paddling experience, confidence, and gear. If you have any doubt, try Deer Lake first!
Also: Did this video get you thinking about Pitt Lake?
https://www.youtube.com/watch?v=mKU5A86l-1kThe title says it's Pitt Lake, but it's actually Pitt-Addington Marsh. The marsh is comparable to Deer Lake, as far as difficulty goes. Pitt Lake is next to it, but is a completely different experience.