Running down a hill puts a lot of strain on your joints. You can feel it when you run outside that there is a large shock that goes through the leg when you land running downhill. You land heel fist and the foot transmits a big force as you land. This doesn't happen when you run on flat or uphill. This has to be absorbed and it's your joints that do that.
For this reason if you have a treadmill that does a decline we would advise that you use it sparingly. It's fortunate that they don't usually have much of a gradient because of the problems they can cause. Just like you shouldn't run excessively on a treadmill incline we would say the same for a decline, even though that maximum could be 3%.
By doing some strength training it would help ease any of the soreness from running both downhill and uphill. Your tendons that connect your tissues together get strengthened as well when do weight bearing activities and anaerobic exercise. We have some strength training routines, they don't take long and can make a difference.
The summary of all this is that just because a treadmill has a decline it does not make it any better than the others. If you have a choice then go for the one without if the price is more favorable. It really makes no difference to your exercising. With inclines going up to 15%, a decline of 3% isn't so great that you should look for it when you make a purchase.