I think optical is more reliable as many laser mice have some issues with skipping and things like that. I had a lachesis laser mouse for example and the cursor really skipped from time to time, used it one the exactmat mousepad.
If I had to decide between the mx518 and DA I would definetly go for the DA, but maybe thats just my personal preference...I don't like logicrap in general.
I'm using a Xai at the moment and I'm really satisfied with it, it's the best mouse I ever had. Really.
The Xai has a pretty good laser sensor, I didn't have any skipping issues or anything with it. It is precise however you configure it.
So if you're running the Xai with low DPI you can use it as a lowsense mouse as well.
I run it at 1600DPI, 975 polling rate and it tracks nicely.