If you go to the Seattle RV show, you can see several brands of truck campers and models at the same place.
Length or a slide will add weight.  In the case of a slide, 200 to 500 lbs.  It’s not more because you lose cabinets which are weight.  Likewise, length adds weight of 200 to 500 per foot.  More length also means more overhang, more overall length, and requires the use of a longer more specialized hitch.  Also, a longer slide needs more support because it is more weight and is harder to support in a small RV such as a truck camper.  
In our case, the second slide does add more floor space which is helpful if you ever use a dog crate.  It also adds to the esthetics which my wife loves.
We can say what we like about each of ours, but ultimately, you have to trade off something.