The simple answer is no, they are not. Are they more compatible and future-proof, sure. This is just a personal suggestion the one I would do so please don't follow through just because I said so. But if this was the case, I would replace both drives with a pair of new ones. Would those be Syno drives, yes (but that is just me), still, you can use any compatible drive for that model of NAS and those will work fine.
If you decide to swap out a single drive, again, personal preference, it would be to go for the same model, or at least as said before, be sure to now cross 5400 and 7200 drives in a single array.
Syno drives from the 3300/3310 are not any more expensive than WD or Seagate drives (maybe 5-10% at best) and from a compatibility standpoint, it would make sense to go for them. Saying this mainly as you are running an x18 NAS that at a certain point in time might be replaced. If and when that happens using Syno drives in the future model will give you no issue, while running with 3rd party drives will. The main reason for this is that most x22 or newer models have a shorter 3rd party drive list with each new NAS.
Now, atm, you will be fine with non-Syno disks for sure, but long term those will become standard for the brand. Again, this doesn't mean those drives are better than other brands, and you will pay a bit extra for them, but Synology is going down a route of full appliance, so at one point this will probably not even be a choice.
As an individual who has both WD or Syno drives (in separate units/pools) I can say both work just as well, so it is down to budget, compatibility, and the warranty imho.
