it depends on your intended uses. when it comes to the majority of applications and windows uses, access and seek time/performance will have a much more profound impact than increased STRs will.
such as, if you play games, youll practically reduce load times as much as possible (your cpu and such would begin to be a bottleneck more), and gameplay should be very smooth as a result too from a data accessing standpoint (similar to caching it entirely to system ram, as thats essentially what an ssd is, but its not volatile like ram is), whereas 2 15k's in raid 0 wont be much faster at all than a single 15k most times, and at other times theres no improvement at all to speak of over a single hdd.
from an absolute standpoint, the access times of an ssd will be less than .1ms, due to no moving parts needing to locate data. whereas a single 15k will have access times many times slower, around ~5ms, and a second 15k in raid 0 will have access time performance slightly slower still, due to needing to position multiple heads when finding data, instead of just one head, its not slowed enough to notice though, most likely. ~5ms is still faster than the ~8ms of a raptor, and much faster than the ~14ms of a 7200. PMR has managed to reduce the effective seek times of 7200s though, due to greatly increased density per platter, resulting in improved desktop performance, putting them closer to that of a raptor (the actual access and seek times are still the same speed though, due to using the same rpms and 3.5" platters, so it still takes the same amount of time to actually locate data, making fragmentation still a very bad thing for them, unlike raptors (higher rpms and smaller 2.5" platters), less so still for 15ks (also 2.5" platters) and not at all for ssds (no platters or rpms to worry about).
when it comes to STRs though, ssds are lower usually. maxing out at about 60MB/s for some current models, and over 100MB/s for other more expensive ones.
another plus is that they generate no noise, little to no heat, consume less power, and arent at risk of failure, again, due to no mechanical parts being able to be worn out, so they have a rated MTBF much higher than that of mechanical hdds too. the downside though, is they can be very expensive, no doubt, mainly due to them being a newer technology.