If you can afford it, I would recommend building the PT-40 over the PT-20. Larger planes are easier to learn on because they are: 1. easier to see, and 2. less affected by wind. The same radio gear can be used for either, but you'll need a larger engine for the PT-40.
The PT series of planes have been around for a long time and they are very good trainers. The design is slightly dated, however. The wings were designed back before buddyboxes were common. Instructors and students had to pass the transmitter back and forth, so having a plane that would right itself to wings level flight was a good thing. Today, buddyboxes are the prefered method of teaching. This makes the need for that high dihedral wing obsolete. IMO even the lower dihedral option in that kit is too much. More dihedral makes the plane harder to taxi without tipping over, especially in a crosswind and , quite frankly, just doesn't look very good ( though looks are secondary to function in a trainer plane). I've suggested to new builders to use half the dihedral called for in the "sport" wing option. Joining the wing halves at a lower angle,however, requires the builder to make new wing joiner parts so your woodworking ability should be taken into account before following this suggested modification.
It should also be noted that this is just my opinion. I'm sure others will be telling you that you should follow the directions exactly and will have a multitude of valid reasons to build the high dihedral wing. That is why you should discuss it with the person or people that are going to be teaching you. Build the plane the way they suggest.