Well, 5 HP might not seem like a lot, but look at it this way.
5HP of 35HP is about 15%. Again, 15% may not seem like that much (seems like a lot to me....)
To look at it from one more direction, if you were buying a car and could get a 150 HP engine, or a 173 HP engine, (a 15% increase), which would you choose?
You see where I'm going here - basically I don't think I've ever heard of anyone complaining about having "too much HP" and wishing they had "saved the money instead" - BUT - I have heard the opposite where someone comes back and said "I wish I had bought a little more tractor."
I'd say if you can swing it financially, opt for the bigger engine.