Totally depends on how you use it.

128gb is more than enough for apps, office documents and a few photos and a few iTunes downloads of movies and music.

It's almost certainly inadequate for modern games*, if you watch a lot of downloaded rather than streamed video**, or if you're a really heavy photographer*** or have a huge music collection.

(* which can easily break the 20gb mark.)
(** although it's manageable for casual use; figure about 5gb per movie -- maybe less in heavily-compressed format, but I'm not sure how heavy a compression iTunes uses.)
(*** I've got 204gb of photos on my laptop, and I know people with terabytes.)

Given that it sounds like you look at the laptop as "extra" with the desktop as the main machine, my bet would be on 128gb being OK.