Unless you find a special price on the 2hp, I say get the 1.5hp or go for a Powermatic.
I have a 2hp Powermatic and have spent 5 days turning on a 1.5hp Jet. During those 5 days, I never felt the Jet needed more power.
The cost difference between the Jet 1.5hp and the Jet 2hp far exceeds the actual cost difference between a 1.5 and a 2hp motor, which is the only difference between these machines.
More than anything, I believe the decision should be based on intended projects and turning style.
Project-wise, if you plan to be doing much turning at the upper end of the 1642's capacity, you may want to consider the 2hp model. IMHO, if your expectation is to turn 14" or smaller bowls and hollow forms, 1.5hp is fine.
Style-wise, Mike Mahoney casually stalls our club's 2 hp PM 3520 because he likes to remove material at a break-neck pace (>3/4" wide shavings!).
IMHO, 1.5 HP is a good balance for the 1642. It is a sturdy machine which I would expect to last a lifetime of enthusiastic hobbiest turning. However, if your expectation is to routinely put really large/heavy blanks on the lathe, you probably can use the 2 HP. The problem I have recommending 2HP on the Jet is if you really need 2HP you should probably be looking at the Powermatic. It is designed to handle the "abuse".