It's two different experiences.
Downtown you can visit 8 casinos in 2 short blocks, under a covered canopy with security. Plus several more casinos 1-3 blocks away in different directions.
There are hourly light shows under the canopy at night, bands on 2 stages, and other entertainment, including one of the best side show type acts I've ever seen - 3 motorcyclists zip around at top speeds at the same time in a tiny metal ball.
On the strip, it's about 1/4 to 1/2 mile from one casino to the other in many cases, but many of the casinos (not including the Stratosphere) are much higher end than downtown. The Bellagio, especially, is worth walking thru just for the visual experience.
Downtown is mostly plain wrap, much of the strip is high end, fancy, and picturesque.