Depends... how skilled are you with a paintbrush?

Paint over varnish is usually a safer option than varnish over paint.

If you're skilled with masking tape you can achieve what is essentially a zero-overlap butt joint between paint colors, or where varnish ends & paint begins.

As always, thorough & proper preparation is a 99.9% guarantee of a job well done.

   Okay thanks for the prompt response. Another question- interlux says to sand with 80 grit prior to applying- is that true for the hull which I have ever so carefully sanded smooth using 220 grit? I have decided not to use pre-kote. 

+1 on spclark's note about paint over varnish. I was all set to paint the hull of my shearwater sport first, but after checking this forum decided to varnish the deck first instead. I was really glad I did. While I was very careful about masking off a large area of the deck I still had a couple slips that got some paint on the deck. Thankfully, it just wiped right off the varnish. If the deck had been sanded epoxy I'm sure I would have had a much more frustrating experience cleaning up these mistakes. Getting a clean paint line with the fineline tape was also easy over varnish.

Regarding the grit for the painted hull, 80 does seem too coarse. I think I went to 220 before applying Interlux Brightside. Are those preparation instructions that recommend 80 grit for the varnish or the paint? My Interlux Brighside instructions actually recommend at least 220.