You did not mention if you were coming from the North (Southbound on I-15). If so, as others mentioned, there is no real advantage to taking I-215 (westside belt) Plus where I-215 joins I-15 it is at best exciting. There are 3 ramps that join there, 1 from the westside 215, 1 from the eastside 215 and a I-15 off ramp. They all merge, with one lane becoming an exit only lane at 7200 South. Lots of traffic "lane diving" back and forth. Plus you still have to merge back onto I-15 in a very busy area.
If you are coming from the East (I-80 westbound) you could take the east side 215, but again it still merges in the cluster of ramps.
A further thought if you are using westbound I-80.
There is major road construction just past exit 164-Coalville Utah to the junction of I-80 and US-40- Silver Creek Junction. The freeway is down to one narrow lane in each direction separated by jersey barriers. A lot of truck traffic travels this way, thus slowing traffic. (On a recent trip through there a semi-truck broke down. There was only enough room for small cars to squeeze by plus they then had to totally stop all the traffic while a tow truck came the wrong way down the road to move the truck.)
After that you have to deal with all of us awful Utah drivers going up the summit of Parleys Canyon, and back down. Everyone thinks they are not the slower traffic so you get a lot of people slowing traffic, or passing on the right, or not letting you in, etc.
If it was me, I would take the I-84 exit from westbound I-80, it's about mile marker 168. This route will add about 12 miles to your trip, but avoiding the construction, Park City traffic, Parleys Summit grade and traffic is usually worth the slightly longer drive.
I-84 westbound travels through some nice country, and it is not as steep as I-80 at Parley's Summit.
At about mile marker 87 is an exit to US89 southbound. Take 89 south. This is a limited access road, but there are traffic control signals. It eventually joins I-15 south bound. If you take this route watch your speed. The Fruit Heights police frequently set up radar speed traps (personal experience of myself and at least a half dozen friends)
Most of the southbound I-15 road construction is down in Utah County (Orem, Provo etc)
UDOT has a smart phone app that is helpful
http://www.udot.utah.gov/main/f?p=10...T,V:1673,57449