I've used both Monokote and Ultracote.

Used monokote on my first couple of planes because it is so common. Then switched to Ultracote on reccomendations that its easier to work with. I definately find Ultracote easier to work with - its easier to seperate the backing and shrinks better. I have found though that the seams seem to peel up more, it doesnt stick to itself as tenaciously as monokote (and I do work forward and upwards so all seams are away from the wind) Also the matching ultracote paint does not seem very fuel proof - one cowl I had to repaint 3 times because of fuel softening the paint, then finally switched to lusterkote white (close enough) and its still perfect after a year.


Monocote
Pros - Edges stay sealed, seams less visible. Better color selection. More fuel resistant paint (lustercote)
Cons - Colors bleed if not careful, doesnt shrink as good. Backing is a royal pain to seperate.

Ultracote
Pros - Very easy to work with, shrinks great
Cons - Limited colors, tough to find good fuel resistant matching paint. Seams tend to peel over time


Ive settled on using Ultracote on most of my sport planes, and Monokote for giant and scale projects that need lots of painted trim.