I’ve not used either of those trackers myself but from reading reviews of others, I don’t think there is much of a difference between them. I hear more positive reviews from the Sky Watcher but that is most likely due to Sky Watcher having a wider availability in Aus than Vixen products.
As for a “fast lens”, it is simply one that works well at wide apertures. I have a Nikon D810 myself and they are really taxing on optical quality, especially for astrophotography. I am yet to do any astrophotography with my D810 yet but I have done some with my D7200 and a Sigma Art 85mm F/1.4 @ F/2.8.
Large Magellanic Cloud
Rho
18 Panel mosaic of the Milky Way
Although I haven’t tested it yet I’d likely want to use F/4 on the D810 to try to correct the coma further off-axis.
Soon I’ll be trying with a second hand Zeiss 135mm f/2 and the D810 at different apertures just to see how it performs.
Obviously you can use wider angle lens like a 50mm or 35mm or even a 24mm but I’d you are using a tracking mount sometimes it is nice being able to isolate a constellation or two rather than ultra wide areas of sky.