as mentioned by horns, my mobo only support sata 2. trim is enabled

Dang, I dont like putting more cables inside my already full small case. Bad cable management some more    . Can only hope for the bigger and cheaper ssd to come out fast then maybe the pcie ssd will come down too then.

in term of raw performance, RAID 0 are awesome. but for general usage/gaming, performance different is negligible. if you already have 2 identical ssd, for the sake of science no prob go with RAID 0. indeed RAID 0 will increase risk of data losses, but it still far less compared to standard HDD.

however, 2x120GB SSD in RAID 0 are not recommended IMO. might as well go with single 256GB SSD. will give you better price/size.

imo it really depends.

1. in terms of costs, while m.2 pcie ssds are still expensive, you can opt for much cheaper raid0 to achieve better speeds.
2. in terms of reliability, it's a give-and-take. raid0 always have risks. previously i used raid0 for os/app/game drive, and store data else where. the risks of data loss is really relative and subjective. i had zero issue.
3. in terms of performance, if a single sata ssd is more than enough, you probably can just stick to that. depending on usage, the difference might not be significant.

edit: my point is, find balance of the above based on your needs. for instance, in my case, i have removed all hdds from my gaming rig. i find the speed of sata ssds alone is damn great for me already.

on a side note: i really suggest everyone get the cheapest ssd that one can afford, at least for os and common software. recently i had a chance to configure some laptops with stock hdd. man, things that i can do in minutes or under an hour, had become hours. many long hours. although hdd is still good for light common usage, once you go beyond that, your machine is seriously bottle necked.