I don't know about the -R Mitsumi, but the 4X +R Mitsumi that I have burns VERY poorly. The -R may be OK though - it really depends on the media code and whether or not the BenQ drive knows about it. Perhaps others have tried this and can give you better advise. I'd consider this one the most risky, though.
I also have +R BenQ media (bought from FutureShop a few months back). Your 1620 will likely burn these discs fine - but only at 4X. In my opinion, you should be a bit careful buying large quantities of any 4X media for the following reasons:
It takes 16 minutes to burn a full disc and the quality most likely will not be any better than 8X media burned at 8X or even overspeeded to 12X.
There's limited room inside the BenQ firmware for media support, and newer firmware upgrades will be required to support newer disc formulations. So support for 4X legacy discs is likely to vanish over time, and a future firmware upgrade could make your discs unwritable.
There doesn't seem to be much of a cost advantage. In fact, for the first time, I'm seeing higher prices for 4X media now than for 8X media. And the prices for 8X media are continuing to drop, so soon you'll likely be able to buy them for less than the "boxing days" cost of 4X media.
Unless you plan to do a lot of disc burning, you'll have these discs lying around for a very long time. And if you do happen to do a lot of burning, then you'll likely want to write them faster than 4X.
Given a choice, it's usually best to go with the +R media. While there are definitely exceptions to this recommendation, and it definitely depends on the media you choose, your odds of a successful burn on the 1620 are a bit higher with +R, in my opinion. Also note that there's lots of bad media out there, and there's media out there that your BenQ doesn't properly support, so it pays to be a bit careful.
Legs2sag hit it dead on. DVD Shrink and DVD Decrypter are two EXCELLENT programs when it comes to backing up movies. Highly recommended!