Welcome back to MUSHing! It's been refreshing to see a lot of older players coming back, even if it's just to see if stuff is still kicking.
All three games you are looking at have pros and cons, so it really circles around what you are looking for IN a game, outside of basic activity. I play/have played recently on all three, so I'll try to break them down the best I can!
Pernworld: As wyrenyth stated, PernWorld is a good beginner's game. So long as you stay outside of the political aspect of the game (which really only matters to staff members or goldriders), it's a very good game. Evenings are when the game is hopping the most, with probably at least a scene per area. The Weyrs are really the only active places, right now with Fort being in Search. Ista has a small, but dedicated core of players always looks for RP, while Xanadu just had a leadership change so there is some excitement happening over there. Western is... there (sorry Western people, I have no idea wtf is going on in your Weyr). During the day there isn't much RP to be had, but a good amount of channel chat since some people log on from work and what not. You'll run into more... well, newbiesh RPers on PW, which can be good or bad. That isn't to say there aren't some very amazing RPers on the game, cause there are, but the trend towards the new players is there and there is everything that comes with that.
As far as if there is any magic around PW due to their setting? It is generally ignored in many areas. There are cell phones and computers on PW, and a lot of people just tend to balk at that (myself included!) and prefer to stick towards normal Pern, with a twinge of technology. Fort embraces the technology the most, I believe! So that'd be a good starting place if you have an interest of Pern with a twist.
The downside to PernWorld is that each Weyr generally exists independently of each other. Due to a lot of political issues in the past, Weyrwoman hating Weyrwoman, the Weyrs do NOT really interact with each other. That's not to say they can't... they just don't. It's almost like each Weyr is a game by itself, and each new person who logs on is a prize to be won for an area. There is hope that with many of us now being and acting like adults, this'll change soon.
Harper's Tale: I'm admittedly biased about this game, since it's my #1. That being said, wyrenyth was again correct in saying that HT tends to be more serious and gritty. It does tend to call upon more experienced players, however, we get our fair share of new players as well and do try to make them feel at ease. Ista Weyr is a great place to go for new players, due to a more relaxing atmosphere, while High Reaches Weyr is for players looking to really jump in with both feet and GO. Both Weyrs are rather active, at least a few scenes a night, with a few during the day for us daytime players as well! There is a TP going on now at HRW that's been fun and has been effecting all of Pern in some form. Ista Weyr is currently in the middle of their search cycle, with HRW's going to be going on in the middle of the summer. There is currently no Thread due to interval, however there are always discussions going on about when to bring it back. Hopefully sooner rather than later.
While crafts and holds are a little slow in comparison to the Weyrs, the Traders are very active, as are the Harpers! And while some of the other crafts (Dolphineers, Weavers, Healers, Herders, Smiths) aren't as active, they have dedicated staffers just waiting for new players. Plus there is always the ability to be a crafter stationed at a Weyr, if that's preferred.
Dragon's Fire: This is the one game I know about the least, so a more recent player should really come in and talk more than me. I believe DF is the ONLY game at the moment that has Thread. So big score in their favor there, if that is what you are looking for! They have two Weyrs, Fort Weyr and Ista Weyr. Fort, admittedly, seems a little less active than Ista, but both have some great players. I'd put this game in between PW and HT as far as new player friendly. In my opinion, it has a smaller player base than both HT and PW, but tends to be very TP driven, with a few very dedicated players. A lot of people who play on DF, also play on either PW or HT. Tends to be more of an evening game, so far as more players are on then and that's where the majority of RP tends to happen. I'm a daytime player and sadly couldn't get many scenes in (which is no one's fault, people have to be at work!) so it just wasn't the game for me for only that reason. Otherwise, everyone is really nice, the RP is wonderful. I have no idea, what so ever, as to who is in Search or about to be or whatever.
Hopefully that helps a little! Or maybe not. While there is some talk that the Pern is a dying theme, we seem to have a pretty steady player base on all games at the moment, with some older players coming back recently! It's actually increased in activity in my opinion in the last year or so and RP has been easier to find. All Pern games have seemed to shift in gear away from the high school / undergrad college student towards those going on for even higher education and adults in the midst of their careers / parenthood who just want a little time to play and decompress. There's a much more mature attitude now a days, with a more laid back vibe than even a few years ago.
It's a really good time to get back into the game!