Motorised or non motorised slider?

Motorized sliders make it easier to repeat a shot, but you want to make sure that the slider will "feather" it's movement at the start and the end, so the camera does not suddenly jerk into motion and then stop abruptly at the end.

Also consider if you want to record sound close to the slider, as some motors can be a bit noisy for audio recording.

Motorized slideres are mostly used for timelapses. For most of the normal shots I use a non motorized slider because then I can controll the camera completely in order to keep my subject in frame.
If you do some very complicated shots it's better to use a motorized slider with a motorized head. But that has to be fine tuned to the actors movements etc.

I would say motorized because you can use it with or without the motor depending on your need at the time.

Keep also in mind, that the prize for a motorized slider is much higher than for a non motorized slider.

It depends on what you want to do with your slider. If it's narrative only a non motorized seems to be the best choice. Because then the DP can focus on every little detail.
But if you do something like interviews and you need a second shooter (and you don't have one) a motorized slider is ideal.

It depends on what shots you want/need to shoot.
It depends on which motor you use and whether sound is important or not.

Shooting a college or a wedding does not require either one of them. It is all in the style of the shots you want to shoot.

A slider like Cinevate's Hedron can always be upgraded with a motor later.

I used to think it's best to start out either renting something great or buying something just good enough. But, if you're making an investment and plan on using it for years, and if you shoot a lot, I have changed my mind to 'get the best you can', and that means one that can be motorized. For me, I went through three sliders until I finally settled on the Kessler Cine one (I chose that one because a) I got one used on eBay and b) I travel for filming a LOT, and the internal rails mean TSA won't be able to ruin it for a shoot). I also bought the Kessler Second Shooter, which can work with that slider, because we do a lot of interviews and time-lapses. Plus, I like the look of a motorized shot sometimes. Just like finally owning a serious tripod and head, owning a kickass slider and decent moco system has improved my work and my life on set immensely.

Anything with a motor will make noise so bare that in mind as it will effect scripted drama and / or interview work. Un-motorised sliders are sometimes lighter if portability is an issue. I have a motor I can detach from mine. Mostly I use it with the motor as recording on location sound isn't generally an issue for me. It's great to be able to repeat a shot and concentrate on what's in he frame rather then the camera movement. But there's a lot to be said for a steady hand too. You can also pull off the same kind of shots with a skateboard or cheap skate cam from eBay if you have a small surface. All depends on what you plan to shoot?

Go for a Motorised slider, even when you don't want to use the motor you can use it manually as well. If I am shooting a timelapse, I will use a motorised slider move.

Personally I'd like to have the versatility of having a slider that can be both motorized and manually used. I do mostly weddings, so motors can become distracting on someone's big day. You'll probably be ok with manual for promos as well.