Dont Listen to Jibthis....he is just grumpy 
Good question though.  This is a tricky topic that has a few variables.
First, it depends on if you have a custom footbed.  If you are going to have a boot that is 1 size smaller than your actual size, you definitely need a footbed in order to shorten up your foot and help it fit within the smaller liner.
Second, it depends on how much park vs. performance fit they want.  In the park, you basically just need to pad up the boot to prevent shin bang, but you don't need a performance fit because you aren't pushing the limits of the boot normally.  If it is the majority in the park, go with the size bigger because too small can cause major black toe nailage....trust me.....im lookin' at my nice big black toe nail right now.
Finally, it really depends on how big you need the toe box.  You generally want to start with the boot that is going to be the least amount of work.  If the bigger size is actually too big, but the smaller one has a little toe box pressure, go with the smaller one and punch out the toe box.
Personally, I would say the bigger boot is better.  This is from a season of skiing in the Head S-11 (a very narrow liner, more like a race fit), and it was TORTURE for the first 8 days.  After a few kinks worked out, and a lot of attention at the shop (I am a boot fitter there as well, so I get a lot better attention than normal), we now have it perfect...but it was a pain.
I vote bigger one!