well it really depends on so many things. one thing that hasn't been brought up is:

where you want to fly? moving from a parkflyer to a "real" trainer is not only one of skill, but also one of venue. while you recognize the need to join a club, understand that clubs also have restrictions on planes. some are all electric, some have a maximum cubic inch (although most trainers are okay).

second, what is your budget? in other words, if you're on a tight budget and have to buy equipment that you can use in your thirds plane, think about that third plane. if you're thinking about say a 40 size mustang, consider buying a 40 size trainer ARF because you'll need to have at least a 5-channel radio for that mustang (6 channels if you control one servo per aileron). consider getting a quality 46 size engine rather than a budget one that often comes on RTFs.

along the same lines, do you want to go electric? if you have the chargers already and feel like electric is your future, then get the Sig EP trainer because it'll be where you want to go later. get a good charger, invest in a good radio and batteries. start to learn about electrics. however, if you feel you want to fly glow planes in the future, go with a glow trainer like an avistar or the mustang PTS you mentioned and get all the ground equipment you'll need for glow planes.

personally i wouldn't recommend the P-51 PTS unless you are flying off thick grass and need that particular trait of a tail dragger. i say go with an Avistar, get a decent 6 channel computer radio like the Hitech Optic 6 and power it with a O.S. 46AX or a 70 size fourstroke. that would be a great beginner package IMHO!