Obviously there are differences, but most folks interested in Indians are after a bit of that nostalgic look, which the Drifters do offer. Yes, the Drifters are based on the Indian Chief, so a little more of a departure from the Scout than the Chief, but it could really come down to what turns you on in a bike.
The Drifters, like the Chiefs have those really deep fenders. If you don't like the deep fenders, then you're not going to be as excited about the Drifter. If raw HP is your overriding thing, the the 100 HP Scout has it all over the 64 HP Drifter 1500. But I never ever felt like I lacked for HP when I rode the Drifter. Plenty of power, plenty of torque.
Back to looks, another departure is how the Drifter's rear wheel and fender makes the bike look more beefy up front, and small in the back compared to the Scout that looks a little bigger in the back or close to even. Personally, I love the deep fender wells and the way the Drifter looks like a front-end beast. The Scout has a tank that curves down more at the front. I also owned a Kawasaki Mean Streak with a similar tank look that I liked a lot.
One thing I can absolutely tell you is the Kawa's cost less, do not take a back seat in quality, are extremely reliable, tend to require less maintenance, offer great performance and are highly customizable...particularly when one considers the thousands of extra $ one saves in the purchase price. Definitely not a case of, "you get what you pay for" unless you are of the camp that values, "name on the badge" more.
Arguments can be made for either. To me, it boils down to what matters most to each individual. I offered my opinion based on my experience, and on the chance the OP might be attracted to Indians more than just the Indian Scout, particularly since he is looking at a used bike.
I later wound up with a Kawasaki Vulcan Mean Streak, not because it was my first choice, but because my Drifter got totaled and I didn't want to spend more than $5k to replace it. So, I was flexible. Best thing I found for sale at the time was a used Mean Streak. At first I wasn't wild about the looks, but I didn't dislike them, and the specs/reviews were great. It wound up being my favorite bike by far, and the looks of it grew on me, especially after I changed out the pipes to some V&H and added a Corbin saddle.
Side by side pics (Scout, Drifter 1500, Drifter 800):
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