I have had a little experience now with both and can offer my 2 c; take what you can use. I currently own, have just recently rebuilt, and am learning to coax a well used Aries servo pendulum gear. My last boat was a small cutter that was set up for sheet-to-tiller rigs. I have also read and studied "Self Steering for Sailing Craft" by John Letcher, which is the most comprehensive book on the subject that I know of. http://www.amazon.com/Self-Steering-Sailing-Craft-John-Letcher/dp/0877420424
Hard to get ahold of, but perhaps you could Interlibrary Loan a copy (I let mine go with my last boat).
I second Cpt. Smollett's suggestion that you try both. No matter what self-steering system you eventually choose, it ultimately comes down to getting various forces to balance out in the way that you want.
Windvanes are (semi) sophisticated systems that, once set up properly, allow you to conveniently manage those forces. They shouldn't be treated as a magic bullet that will allow you to ignore trim and balance. To get the best results, you will need to take into account many different factors that will affect the way your boat moves and take pains to ensure that your setup is free of friction and perfectly symmetrical/balanced.
Sheet to tiller rigs are a step up from steering by sail and tiller balance alone. They allow simple mechanical feedback and can be set up in quite a few different ways, depending on your boat's rig and your point of sail. While less convenient than a windvane setup, they are more versatile and may even out perform them on some points of sail. For instance, many servo-pendulum gears do no have enough sensitivity for downwind sailing in light air, while twin jibs with sheets led back to the rudder will stay on track in the lightest of breezes. The downside is that while simple, the different setups require much trial and error to perfect and also have to be re rigged every tack and course change.
If you have the money and the inclination to do the job right, the windvane is the obvious choice. Stick with sheet to tiller if you want to take it slow and learn as you go, or aren't ready for the commitment that doing a good job of your windvane installation will take. The convenience and reliability of a properly set up system really is a wonderful tool for the long distance sailor, but it needs to be done right or not at all. Because I started with sheet to tiller rigs, I learned about the importance of sail trim, balance, feedback tension, friction, and how to manipulate them to make the boat do it's thing. This has really helped me deal with my janky old Aries, which my girlfriend has dubbed "Flippy". After a partial rebuild, he will do the job if all the conditions are right, but he would cause major frustration to someone who just wanted him to "work".
So, maybe what you should take from this is that while a new vane gear might be pretty expensive, saving money on a used one might not be worth the headache if she doesn't perform "like new".
Also, some of the same hardware is used for sheet to tiller rigs as windvane rigs. With a snatch block on the inner side of each cockpit coaming and a pair of opposing cam cleats mounted on the top of your tiller, you can reeve a control line from your vane gear or simply attach a bungie and a line connected to your mainsheet (read more about sheet to tiller rigs for the specifics). My Aries has a small chain and a sort of chain-hook thing to connect the control line to the tiller, which I dislike immensely as it keeps trying to pinch my fingers off.
You can see here that I removed that chain poop and just have a small piece of line tied to the tiller with a constrictor knot. To tension the line, I used a beefed up rolling hitch so that I could slack the control lines to remove them.
First chance I get, I will be going back to the cam cleats, which allow easy tensioning and removal of the control lines. Best way to go, IMHO.
Anyway, that's quite a wall of text, but to sum up, here is my advice in bulleted points:
-read "Self Steering for Sailing Craft" by John Letcher
-learn about boat balance
-set up and practice sheet to tiller
-get a windvane, preferably new, or like new
-if you get a used windvane, you not only save money, but get to practice the finer points of engineering boat balance
-since your setup is upwards/backwards compatible, you can use sheet to tiller if things break, or you want to outperform your windvane on another point of sail