This is a seriously tough choice. Both Derek Yu and Lucas Pope are developers I really respect and who are an inspiration to me as an amateur game developer, and both of their games I followed closely throughout development; I still remember playing Spelunky's early builds, and there's absolutely no doubt in my mind they both deserve my money and more (I get the feeling many share these thoughts though, and both games will have significantly warm welcomes to Steam so my mind is at ease).
On the other hand Guacamelee's devs I haven't really followed for long, but I liked all the announcement shenanigans and the "NeoGAAAAF!!!!" vine they did, so I already like them
plus Guacamelee looks nothing short of brilliant, and I'm a huge Metroidvania fan. However, I know for sure my brother is getting this one and I guess that will allow me to play it enough so that I can wait until it's on sale or I come across some extra money.
I think I'll go with Spelunky, simply because I've been recently craving a good roguelike (missed out on Rogue Legacy, although it still sits firmly somewhere in the top spots of my wishlist), and because I've been hyping myself for it this past week, watching LPs and reviews and gosh, I was so damn frustrated when it was announced it would be a Xbox Live exclusive
I'll eventually buy Papers, Please though, and I'm crossing my fingers it will be sooner rather than later.