Light vs heavier 5ers

Be sure you know the difference between `light' and `lite'. I would ignore the model description `lite' and pay attention to actual published weights. Think about where the reduced weight might come from. Thinner structure is not a desirable weight saving option nor are thinner countertops or roof materials; just to name a few.
X2 on finding the rig you want then buying a truck to tow it.

A heavier unit will more than likely have more features and a higher payload capacity (ability to carry more weight). A heavier unit will require a more capable truck.
Durability and longevity are not necessarily a related to weight
Initially you can shop all trailers and decide which you truly like, however at some point you will also need to determine how big a truck you want. Are you prepared to purchase a 1 ton dually pick up?
The truck and trailer are a matched combo, as you narrow down your 5'er choices and sizes you will begin to figure out how much truck is required to haul it.

It is difficult to provide definitive answers to general questions...light does not necessarily mean flimsy and heavy does not necessarily mean durable. We have found that the more time we spend in a particular unit, the larger we want it to be. Will a larger/longer fifth wheel be more challenging to maneuver? Probably. Since you are in the decision making mode, you might consider finding a fifth wheel that meets your needs and is one you like...then start a search for a truck capable of towing it. As azdryheat pointed out, your fifth wheel will have its own breaks and should be able to stop itself.

I'm comfy with 32' fifthwheel that's 13,000lbs and a '04 F350 SRW SC LB 4x4. But I would prefer a '36 class A Moho with slides and a 2017 four door Jeep Rubi, fully modded.

No TV yet.

Re: being pushed down hill. Your trailer will have brakes and should brake itself going down hill - should, assuming they're properly adjusted.

What type of TV do you have? That will determine if you can rule out a heavier 5th wheel and look for a half ton towable. Durability will depend on make and model of the 5th wheel you are looking at and you have to take into consideration how you intend on using it. Some have great luck with a light weight but others who may be travelling further or using it in cooler temperatures may require a heavier unit.

The newer models of one ton DRW trucks of any of the "Big Three" are well equipped to handle the heaver 5th wheels. Even my 6 year old F350 comfortably handles our 15,000 5er. Yes some of the newer 5er's weigh more but the advances in technology allow higher weights. Also many of the newer heavy fifthwheel trailers have disk brakes which improve the stopping over drum brakes.
Length more than weight will affect maneuverability. Turns will be wider with a longer trailer. Ever notice the signs on trucks stating this vehicle makes wide turns.