Kenneth, the board is also wider, not just longer.
I agree with the guy at the shop - I'd say go with the 140. To elaborate on what Scott touched on - the 135 will sit deeper in the water and have more drag so it will not be as responsive as the 140cm.
Like Scott said the bigger board will help you learn quicker as a beginner.
I don't know about the Hydroslide brand, but you can get a Liquid Force Rhythm 141cm setup with 2001 Super suction bindings for $259.99 from a certain online watersports retailer who's name sounds like Rovertons
. That's not a bad deal at all on a good starter board w/boots.