Trying to decide on buying the Gotama 170's or 178's. I'm 5'8", 150lbs, ski about 20 days per year, and am moving on from a pair of '07 171mm 4Frnt MSP's (92mm @ waist).

I ski resort runs at a very high level. I am getting more and more comfortable skiing the park, pow, and trees with each season.

So given I want to be nimble enough for the trees and have previously skiied 171mm's (although I've improved over the past 3 years), is it worth going up to the 178's or should I go with the 170's?

I'm 5'9", 160# and skied 170 last year, demo. I actually weighed ~145# at the time and thought the 170 felt good, but they do ski shorter than their length due to rocker. They're fairly stiff up front, so that helps stability. We're talking just a bit over 3" in length difference, so not a huge amount. I've actually just purchased 178 and getting mounted up, but keep thinking I should go back down to the 170 length I demoed. Either way, you'll be fine. The longer will weigh a smidge more, but be a bit more stable at high groomed slope speeds. They'll also float better and keep the tip up better in pow. The shorter length will appear and feel more nimble through trees and likely be less intimidating visually when putting through tight spaces.

The only reason I can see that you'd want to go up to 178's is if you're worried that the 170's won't be stable enough at High Speed, or won't provide enough flotation in pow. At your size you're probably fine with the 170's, especially if you never noticed any chatter on your MSP's at high speeds. 178's would provide more flotation in the pow, but the 170's will be more maneuverable in the trees, and easier to spin in the parks.