Bobbysamd, if it weren't for your apparent love of the Denver Broncos, I'd buy you a beer next time you are in Seattle. I read and agree with most of your posts, keep up the good work.
P.S. GO SEAHAWKS!!
alaskaairlines-If you are not an instrument pilot, you will NOT have to shoot approaches in the multi. You will only be required to perform the basic maneuvers and demonstrate proficiency in single engine operations.
Once you complete your initial instrument rating in the twin, you will be instrument rated in a single and a twin.
The same is not true for the commercial, you will have to take two different checkrides. However the single engine add on rating for the commercial is widely thought of as the easiest checkride you can take. Since you have already demonstrated instrument proficiency in the twin, you will not have to shoot any approaches on your commercial checkride either.
As far as the instrument ground school is concerned, since they are all on CD_ROM, you will not lose any information by waiting two months. Get your multi rating.
The cost difference between the 172 and the Duchess is more than I would expect, but the same economics still apply. Getting your multi now will put you in a better position at a cheaper overall cost. Think long term, most pilots in your position don't think past the next lesson. If your goal is to fly for an airline, its all about the multi-time. Good Luck