Which is better, Raid 0 with two hdds or 1 ssd?

An SSD is almost always going to be faster than spinning platter hard drives. (Perhaps, maybe, possibly, if you have a dozen hard drives in RAID0 then maybe you could get as good burst throughput as SSD.) And realize that RAID0 is "dangerous" in that if any one of the drives fail, you lose all the data on those drives.

So "better" is relative, but for a notebook computer, where you normally would have 1 HD and can't usually have more than 2 internal drives, RAID0 is usually not the best choice, particularly if you're going SSD. (Slightly different comparing RAID0 to a single non-SSD hard drive or SSDs in the RAID array. But that is not what you were asking.)

I understood what the hard-drive-in-optibay meant. Regardless, it's still two platter-based hard drives versus an SSD. An SSD will be faster in your situation. If you had a lot of platter-drives in RAID0 and you had some sort of hardware based RAID controller with battery-backed cache, then maybe, possible, potentially, that (really expensive) platter-based RAID0 could be as fast as a SSD. You don't have that and can't put that in a notebook, so it ain't gonna happen for you.

Now add to the mix, the fact that RAID0 will lose data if any one of the drives in the array fails. You have two drives in RAID0 versus one drive in a non-RAID. The RAID is twice as likely to have a drive failure just because you have more drives. If you made the RAID from two SSD drives, which are more reliable than platter-based drives, then you're got something interesting. But you're still twice as likely to have a SSD-RAID0 array failure compared to a single non-RAID SSD. (And more than twice if it's platter-based drives in the RAID0 versus a single non-RAID SSD.) So it's up to you to determing how much risk you are willing to take...and how much you can afford.

Also note that some Macbook and Macbook Pro models have slower performance on the SATA port for the optical drive. (One assumes Apple tried to save a few dollars and put in a older, slower SATA controller for the optical drive.) I don't know exactly which models but if yours is one of those, then you may get uneven performance on your RAID0. Whether that is noticable or acceptable to you, will depend on a lot of things that you won't know until you try. (e.g.: model of drive, apps you're using.)

And it's good that "backup is easy for you." With RAID0 for the boot volume, you're that much more likely to have a failed boot volume. (IOW, can't boot the Mac at all, which means you'll be using that easily made backup a lot. 😉 ) Ideally, workstations that used RAID0 used them for the data, not for the OS. For example, in video editing, the scratch disk needs to be fast, but could be RAID0 because you "threw away" whatever was on the scratch disk after you were done editing, and presumably saved/copied the final edited product to another archival drive. But the OS was on a typical single drive non-RAID (or a RAID1 or RAID5 if necessary,) not on a single RAID0 array for both OS and data.

In your situation, you can't separate the OS and data onto separate RAID arrays, so in general, RAID0 is NOT what you want, regardless of whether you use SSDs or platter-based hard drives. But again, only you can determine how much risk and cost you're willing to gamble with.