I just bought a Rasberry Pi and I want to turn it into a media center, but I dunno if I should use SSD or HDD, I know SSD is faster, but HDD is cheaper so.
If it is a local media center that will not stream multiple HD streams at the same time from harddrive, any modern hard drive will have enough speed for media center applications, so if cost is your consideration, go with HDD. Morover, your data transfer rates will be more constrained by USB2 speeds, which are generally slower than HDD, so SSD or SSD won't matter unless you figure out direct SATA connection, like with X300 extension board, but with that board I'm not sure you'll be able to pull real SATA2 performance to benefit from SSD speeds.
BUT there's one very good point for SSD, even at slower speeds, there's NO moving parts. RPI does not have any moving/rotating parts like fans, so if you want silent operation for your media center, go with SSD, you can't get quieter that that. Plus HDD have more power requirements, so you can say SSDs are greener to operate.
I was having the same idea using Raspberry 2 and 3, but in the practice I found a series of unexpected results.
First. On the RPI3 both have similar throughput because of the USB 2.0 bottleneck. When I write a one million character text to disk it needs around 319 milliseconds, and on several tests on a 5400 RPM disk and a Z400 SanDisk SSD, they interchange who is better. So, the speed is not the key factor to choose here.
Second. An isolated media center (one WIFI with one HD) works well. But when this is also connected with the ethernet interface to another machine, the throughput is terrible. This happens because the machine only has a 480 Mbps controller for all the I/O and to try to read from the disk while sending on the ethernet and receiving requests on the WIFI actually kills the Raspberry (and this happened to me with both, model 2 and 3 using different types of WIFI dongues and the integrated RPI3 interface). So, in this respect, could be better to "share" responsibilities between several Raspberry, all then communicated on the same WIFI network.
Third. Always a 811.n will be faster than the constrained 100 Mbps the Raspberry has. The first one can use all the 480 Mbps if needed, but the other one is limited to 100 Mbps. Take this into consideration, and use the Ethernet only because "other" reasons, not because speed.
Fourth. Oh yes, configure correctly your WIFI network and check networks are around to avoid having problems because of external factors.
In the answer 1 looks like hdd requieres more power than SSD, but that's not truth. While 2.5" HDD Western Digital Caviar 1TB requires 0.60A a SSD disk could require 1.5A or 2.0A. So SSD is faster but consumption is higher. Remember it, and also usually the form factor for SSD is 2.5" and SATA connection, so you need also a Case to convert it into an USB 3.0 connection which also requires some power more.