As stated earlier, slide or no slide will depend on you, your plans and your method of camping. What you want matters the most.
We had a small trailer with a slide and I was amazed at the added space. They really open up a trailer and the big boys with 3 slides are amazing. Someone stated that they love their slides because they camp months at a time. If I were full timing I would get slides, the more the better. But, we don't camp full time and don't want to deal with the added cost, the added weight and the added potential for issues. We had issues with ours and while fixable, it wasn't fun.
We like to go down the road and stop without having to worry about putting out the slide. We also don't need the extra room to get around our kitchen island, or to extend our matching recliners so we can see our big screen tv. Don't forget the built in "fireplace" that we need to get in front of. Again, it comes down to camping style. If we had a big family that came with us in our trailer then space would be more important. Again, to each his own. Consider what you want and go that direction.
As far as resale goes, a well cared for trailer with a fair price will sell, with our without slides.