Some more thoughts on it.
A slide in is kinda nice in that you don't have to deal with a trailer. Or you can if you want to, if you play it right you can pull a trailer too. Just like everything else it is all about trade-offs.
Canvas side pop-up slide in
+
Lighter
Lower clearance (easier to fit in a garage/shed)
Neat openess with the top windows open
Catches less wind for both mpg and handling with a crosswind
-
Lift mechanisms can be light duty (operate carefully, don't put a bunch of crap on the roof for it to lift etc)
Roof structure sees substantial stress from lift mechanism (more of an issue with age)
Still tall enough to be a pain to store inside
Harder to install in a lifted truck (I have to let the air out of my 31's)
Less secure in theory for bear country (if a park has a problem bear they will boot softside campers until they get it resolved)
Door is short (but it works better than you think it will)
Careful with the canvas/plastic windows!
Can get dew inside like a tent
Hardside slide in
+
No lift mechanism or canvas to worry about
GTG during "hardside only" camping proclamations at national parks due to bear problems so you won't get booted for being a soft sided camper (keep in mind it is still toothpick and tinfoil construction for real world bear security... don't leave your bacon grease on the stove when you go fishin')
No moisture weirdness associated with a tent
Better in cold
Easier to mount things on the roof (aka canoes etc)
Probably quieter inside (coyotes, wind etc)
Generally have closer to a full size door
-
Catches more wind on the road, both for mpg and crosswind/handling
Heavier
Even worse clearance than a pop-up slide in for drive thrus, garages/sheds
Harder to install in a lifted truck
Prices vary wildly, out west like California or Colorado they can be pretty pricey with the current "overlanding" fad in full swing. In the midwest... everybody goes for a fifth wheel to drag 5 miles to the local state park and/or the state fair so the market is more limited both to buy and sell. They do make hardside popups... I haven't really looked into them.
Now if you meant a popup trailer...
+
Neat open view
Lightweight... Someone makes one you could pull with whatever you have (does not require a truck with a bed between this long and that long that it is no more than this deep with a cab that can't be taller than this...)
Easy to park/store inside
-
More tires to maintain
Another license plate (some states require slide ins to be registered)
Trailer to insure (again some states/insurance companies consider a slide in a separate thing)
Trailer "fun" for maneuverability
Like 0 real security (at least with a pop-up slide in the canvas is 6-7' off the ground)
Careful with the canvas/plastic windows!
And you get a lift mechanism of sorts to take care of
Can get dew inside like a tent
Just my $.02.