I have Kessler's 3' CineSlider, & their 2' PB PocketDolly (which I pulled the labels off). Both these come standard with cranks. While I never manipulate the cranks directly, their inertia helps smooth out the moves.
A good motorized slider is typically much smoother, & repeatable, than manual pushes, but even the most quite motors are often too noisy if recording sound. My preferred method, especially with the 3', is to have an assistant push the sled while I operate the fluid head. If your not panning/tilting while sliding this is less necessary.
The 3' typically takes two people to setup, raise/lower, & relocate, while the 2' can easily be handled alone. But the 3' does provide for a longer move before bumping into either end, which is all to easy to do.
Combined camera, & head, weight can be an issue if you plan on mounting on sticks, & even if I shot bag the legs, I also support the 3' on either end with monopods. Some users prefer mounting sticks on either end of the slider, but I find it easier to raise, & lower on one set of legs. With either length I use a Manfrotto half-ball to attach it to the sticks as I prefer not to attach it on top of my fluid heads, which are only rated to 20 lbs, or so. And given the option I'll skip mounting any slider to sticks, & instead use Apple Boxes as setup is much quicker, but be sure to bring some wedges for uneven floors, ground, or boxes.
Another factor to consider is how high above the slider you'll mount the camera. Typically lower is better, & I generally use a ball relocater which puts the head off to the side, but also carry a dedicated Hi-Hat While the Hi-Hat raises the height overall which can effect stability, it helps clear the end of slider when dollying straight in/out at the subject.
If your not using a Hi-Hat, or ball relocater, & still need a fluid head, then most likely you'll look at Manfrotto head that can be flat mounted, & a leveler below that for Dutching. Manfrotto "fluid" heads are OK at best, but their flat mount is useful on sliders, monopods, etc, so I have a handful of different models that all use the 501PL camera plates.
The beauty of this system is Manfrotto also offers a Quick Release adapter (577) that I mount on shoulder rigs, 15mm LWS baseplate, etc, so I can quickly switch from one configuration to another, & Kirk offers an Arca-style QR release adapter that I use on the rare occasion I need to support a dSLR style camera.www.kirkphoto.com/SQRC-501PL_Manfrotto_Quick_Release_Clamp.html
But best of all the 501PL plates are compatible with Satchler's FSB series fluid heads that are equipped with the "Side-Load" quick release (but not the "Touch & Go"). My FSB-8 head is so much nicer to work with than any Manfrotto that it alone made my operating much smoother, especially sticking the end of a move.
I have a couple styles of levelers below the Manfrotto heads, but my favorites are Acratech's. They very well made, & even the smaller version is very strong. I keep them under all the flat mount heads, including the one attached to the monopod.http://acratech.net/home.php?cat=2
When you fly a 2' slider is much easier to deal with. I've put them in luggage, but typically it lives next to the Satchler sticks in a TuffPak rolling tube case (along with SteadyBags, & monopod), while the 3' goes into a dedicated Storm Case designed for rifles, which can draw the attention of the TSA.
And while it may be impractical for your needs in NYC, I suggest looking at the Dana Dolly. It's truly pro kit, & used on all levels of production.http://www.danadolly.com/
Lastly I'll add whether you truly want a slider. While they can add value to some shots, mine rarely linger long in the edit, & I'll often just make handheld moves instead. Simple shifting the weight on your hips can mimic a short slider move, & handholding can lead to more complex camera movements, especially when time is short. If handhold moves aren't smooth enough for the shot I'll often opt for an EasyRig, &/or slow motion.