The Cub Cadet 1500 Series is a vast improvement over the "Cadet" 1000 Series which is just another generic MTD disposable. The 1515 has the Kohler CV15 426cc motor and 38" deck. The 1517 has the Kohler CV460 460cc motor and 42" deck. Otherwise the two are identical. These are very nice lawn tractors and a major improvement for MTD.... (the other Cub Cadet 2000/3000 Series are still made at the IH plant and are really a very different class/quality than the typical MTD box store offerings).
The 1500 has the most heavy duty tranny in its class and a generally solid build. The only thing I don't like about the 1515 is that the 38" deck is generic MTD and is particularly shallow. I think a deeper deck allows for better cutting flexibility in variety of conditions....ie: heavy/tall grass. For example you will notice that all commercial lawnmowers have deep decks. The 42" deck on the 1517 is a bit better. Otherwise, these are very nice machines.
They compete very well with the Deere LT Series. The LT150H with 38" deck and LT160 with 42" deck.
I think the Deere machines have better decks but the Cub has a better tranny. Otherwise it's a wash and either machine would be a good choice.
Good Luck, Bob