Rhino...this is like comparing a Harley to a Suzuki; its all about what suits you.
The main thing, I think, is what you want to do with it
Space/Room: If you want to ride two people for a daily jaunt, the Venice is fine, but is not nearly as wide, and does not have as much storage behind the seats as the Sling, particularly if you both are going on a trip. One person on a trip would be no problem as you have the passenger side for bags or whatever.
Handling: To me, it feels more stable in curves; but then, I never had any problem with my Sling either. Lots of folks say a two wheel front drive pulls you around, while the Sling rear pushes with its one wheel drive. I'm a senior cit; my hotrod days are over. The Venice has excellent shocks whereas the Sling has lousy shocks and I felt the need to get after market ones to tune of $1000.
Modifications: Tons of vendors selling "stuff" for the Sling; I did the shocks (twice), seats, windshield, wind wings, and various gadgets to tune of 3K bucks. Didn't have to, but they were avail and made the Sling more comfortable for me. The Venice is so new, there aren't any vendors I know of. Factory makes a rear rack, cruise control (standard on Sling), bump shifter, and are coming out with a few other accessories. So, if you like to Mod, you're not gonna be doing much to the Venice unless you have it custom made. I've made a pair of wind wings that turned out nice; dealer thought I'd bought them somewhere. The Venice windshield (glass vs lexan) sloops back too much for my taste as it lets too much air dump on my helmet. I've figured a way to lean it forward a bit with some moderate machine work to mod the w/s brackets.
The brackets are some sort of alloy and should be fairly easy to machine to get a diff angle.
I put an aftermarket tall one on the Sling that put the air up and over my helmet.
My Sling and most others I know of, have a NOISY angle drive unit that converts the drive over to a belt. No one has been able to defeat that whine; bothered the crap out of me, doesn't bother a lot of others. That, in itself, is not a big CON for the Sling. Few people had any mechanical problem with the angle drive although a couple of major breaks were reported. The Venice is still so new, and there isn't a lot of folks on Facebook with much miles on them. I would expect the GM motor and xmsn, like the Sling, which has been out for some years to be a robust unit. Vanderhall makes the boxy frame out of welded aluminum and it appears to me to be much stouter than the tubular frame of the Sling. You sit down in the box.
The Venice has a quiet rumble that sounds good without being loud, and if you hit the gas the turbo moves it out smartly. Auto xmsn is smooth and effortless; I had a bump shifter put on for when I want to select the gear. The Sling doesn't have a nice a sound, but I had no issues with clutching/shifting...the xmsn was smooth. If you wanted a little sound, you'd need to change the muffler which, I believe, is fairly costly due to the way the existing one on the Sling is tucked under. The auto shifter (not the bump shifter) is located between the seats, towards the front part, so if you'd like to move ur butt over you rub against it. If you got a skinny one, no problem! lol
As far as I know, most mechanical parts and electrical wiring is made by GM. Even the swing arm? (things the front wheels attach too) were made for the Pontiac Solstice....very substantially made units. The steering arm with turn sig and such is GM. A slight down side is the Cruise Ct is operated by flipping a toggle on the dash because the wood type steering has no space for such. That could prob be changed but I like the looks of retro style steering wheel. I had my cruise ct put on the toggle nearest me and later put an slight extension on it to make it more intuitive. I use cruise a lot.
Visibility...seeing all around you is excellent in the Venice...not so much rearward on the Sling, even the sides, nor the front which was too easy to bump the lowers against the curb. I had put a curb sensor on the Sling but its not needed at all for Venice as you can easily see where the front is when parking. On the highway with the Venice you can easily see all around you. For that reason, I feel more at ease in town/urban traffic.
Some of the above is small stuff that may not matter to others. Best thing is try them both to find out what suits you best. If anyone lives near me, I'd be glad to let you test drive mine; if it ever gets warm again!! lol. If you have specific questions I'll try to answer them.