Tony Buttler published a 3 view of the V-517, a perspective drawing of the V-517 and a front view of the related V-520 design plus information on it in American Secret Projects: Fighters and Interceptors 1945-1978.
Variations on a theme. Seems the fold out lift engines were a design fashion of sorts in that general time frame when you look at some of the other designs coming out of Republic, Fairchild and the like though you can say the same for the lift engines in the fuselage along the center line. It would be interesting to read what the Navy was thinking and if they had preferences that the two designs were trying to cater to.
I would think that the fold out engine version has better roll control in slow flight/hover and possibly a more stable hover than the lift engines mounted in the fuselage aft of the cockpit. I would also think that general maintenance would be easier with the fold out engines though you pay a price with a bit more complexity in the mechanics of the set up. Either way though, that's not a whole lot of airframe and volume for the fuel needed to make things go for more than a few miles!
Here are two more 3 views for the V-517 out of different documents. You'll note the use of two lift engines vs. three as shown on the first drawings I posted and other minor details/refinements too. The listing of the engines is useful too.