Mary, congratulations on your new Hobie! IMO, an extra mast is an unnecessary expense. First of all, your chance of breaking one with standard or ST fins is VERY small. If somehow you do manage to do so, it's better to return to shore one fined and make the repair at home, at which time you can order what you need. Finally if you should decide to upgrade to the V2 sprockets later your unused spare mast becomes obsolete.
I like the ST fins for you. The performance is good, the price is better and the installation is very simple.
There are a few things you should watch on your V1 sprockets.
1. Check each time to make sure your Allenhead mast retaining screw isn't backing out. If it does, use some Loctite Blue to give it a better grip.
2. Check to make sure your masts don't wiggle. If they do, double check your screw and make sure the flat spot (on the mast) is fully facing the screw (you can mark the location with a permanent marker on the mast).
3. Check for any signs of "wallowing" due to enlargement at the mast hole entrance to the sprocket. If this happens, remove the mast and screw, clean everything up and epoxy the whole assembly together (including the screw).
Eventually you may want to upgrade to V2 sprockets and masts, but there is no need to rush into it as long as the V1 sprockets are doing the job.
V1 sprockets and masts perform exactly the same as the V2s. Almost all parts failures occur gradually as the screw backs out (unnoticed), the mast loosens and starts to slide out, eventually falling out. Along the way it can enlarge the sprocket opening to where the sprocket needs replacement.
Now that you know these things, you know what to watch for and how to prevent problems. I recommend you save your money for something else! In fact, unless you're going on an extended trip, I think it's more productive to learn how to effectively use a "failed" Mirage Drive than carry tools and spare parts. Usually one fin will get you back easier than paddling. Besides, on-the-water repairs with wet hands and a rocking boat with no work bench is tough. If you can do that, you're a much better mechanic than I. The only thing you might need is a set of dykes to cut the split ring that holds a disabled fin on (if you can't remove it by hand -- on the water)!