if I really want speed, SSD is my option.
Otherwise, I'd buy a large hd (from 1 to 3T) which still gives me a decent speed.
ssd's are still super expensive you could raid 0 two of those and I'm sure you wont notice a difference. So $240 you'd have 900 gb for a $240 ssd you would have 128 gb.
I would rather have the SSD. Latency is what matters not peak sequential throughput for most uses. 128GB is fine for putting OS, application, and games (although if I were buying one today I likely would look for 240GB models). 10K RPM creates a lot of heat, noise, and electrical draw but doesn't do much for latency.
For games & OS most of access is going to be random reads and that is where SSD shine.