I've heard all these questions, and some of them assume that it's an either-or choice between parchment paper and Silpats. For some people, it is and either-or. Me, I use both.
First, let's talk about the Silpat. On the plus side, it should last nearly forever if you take care of it properly. BUT. You shouldn't use it if it gets damaged. So you have to treat it a bit gently. You can't cut things on it, and you should fold it or scrub it or mangle it. A Silpat has glass fibers inside, and if it gets cut or damaged, those fibers could end up in your food. Yuck.
Silpats are cheaper now than they used to be. Recently, I've seen them for about $20 for the half-sheet size. But if you plan on using them for your Christmas cookie baking frenzy, you'd probably want several of them. It starts getting expensive if you can fit three sheets in your oven and then you want an extra one or two for the cookies you are getting ready to put into the oven.
But is parchment any cheaper? I've seen rolls of parchment paper of various lengths at the grocery store for under $10. Honestly, though, the rolled parchment is annoying to deal with. Flat sheets are so much better. I've seen little packages of parchment paper in the grocery store as well. These are folded. I've never bought them, so I don't know if the folds are as annoying to deal with as the curls in the rolls, but if you don't need a lot of parchment, I suppose it's nice to be able to buy a few sheets at a time. But it's probably not cost effective if you use as much parchment as I do.
The last time I bought parchment paper, it was in flat sheets, and I think there were 50 sheets. They were cheap. But the last time I was at that store, they didn't have them. So when i got close to running out, I started looking around to see where I might find a better deal.
Online looked like the best option. I found flat sheets in packages of 100 for ten dollars. A much better bargain than either the rolls or the folded parchment. Shipping was an additional ten dollars, but that was still less expensive and more convenient that the grocery store options.
Before I ordered, I decided to check the local restaurant supply house. Now, this was a bargain. I found full-sheet size parchment in a box of 1000 for a little over $40. I figured that I could rip them in half or fold them and use them doubled, and it was still cheaper than the packages of 100.
Then the store owner suggested that I take the box to one of the local print shop and ask them cut in half so I'd have a nice clean cut and I'd end up with 2000 sheets that would be the right size for my half-sheet pans. He said that he'd heard that other people did that and it cost about $5 to cut them.
A friend of mine owns a print shop, and she likes to cook. I brought my parchment paper to her shop where she cut them for me, and I gave her a big chunk of them for her trouble. She was happy and I've still got a massive supply of parchment that should last a long time, even if I get wasteful.
When it comes to using parchment, one advantage is that you can cut or fold it to size. Silpats can't be cut or folded, so you can't use them to line a brownie pan, for example. A second advantage to parchment is that you don't have to wash it. Just toss it, and the mess, in the trash. Silpats seem to attract grease, and it seems to me that it requires a bit more soap and hotter water to get them really clean.
And parchment works well under bread that will be baked directly on a stone. After a short time, you can remove the parchment or just leave it there. I'm not sure if a Silpat could handle the heat of a baking stone. I've never tried it, but I think the thickness of the Silpat would make it less than ideal.
Whether a Silpat or parchment is right for you depends on what you make. If you make peanut brittles and similar candies, you'll love the Silpat. If you bake brownies and sticky quick breads, you'll love being able to fold the parchment to fit. Parchment is also great for steaming in a sealed packet. And if you have enough of it on hand, you'll probably find plenty of other uses.
But there's yet another option: Super Parchment. It's thin like parchment, but it's reusable like a Silpat. And it can be cut to size. If throwing away parchment seems to wasteful to you, or if you can't find cheap parchment and you're spending too much on the small quantities, Super Parchment might be just right for you. I've use it to line pans and on my baking stone and it held up well. But of course, once you've cut a big sheet down, now you don't have that big sheet to line your big baking pan any more.
I checked prices on Super Parchment, and it looks like it's currently selling for about $10 per sheet, which isn't bad. Personally, I like the disposable parchment for lining my baking sheets and for folding to fit square pans. Super Parchment, on the other hand, is nice for cutting to fit round pans. I don't know about you, but cutting parchment to fit the bottom of a round cake pan is annoying. It always takes me a couple tries to get it just right. With the Super Parchment, you can cut once and then store the circles for the next use. It's worth having to wash them just to keep from having to endlessly cut circles.
So now you've got three choices. Which is right for you? Maybe one, maybe two, or maybe all three. The best advice I can give you is to avoid the parchment on rolls. Fighting those curling sheets just isn't fun.