LoveSnow, I'll quickly answer your Q's and will probably add more later...
To your 1st, 2nd questions, I think either is just as good as the other.
3) I believe that Londonderry schools are better than Merrimack, especially the music and arts programs (but that may just be bias

I am after all a Lancer Marching Band FOM MOM)
4) Oh oh..... Public water and sewer. Here's where you decision may be made for you. Londonderry has VERY FEW homes with public water, and even less with public sewer. Merrimack, OTOH, has plenty to chose from, and in very nice neighborhoods. If this is truly important to you and your family, this may be just what you needed to hear to make that decision.
5) Here is another area where Merrimack wins out. Londonderry has been trying to get a semi-public pool put in (via YMCA) but there have been roadblocks (a physical fitness facility has the only large indoor public pool in town). Londonderry does have tennis courts, tons of fields for sports, etc, but that's all...
6) Keep in mind that LEEP (Londonderry Early Education Program) is for 3-4 year olds with and without special needs. This meets Mon-Thurs AM or PM.
Chester and Auburn are limited in their services, and they like it that way. Many votes for enhancements to town services are defeated by voters. Chester HS kids are tuitioned to Pinkerton in Derry. Auburn HS students are tuitioned to Manchester Central.
Windham is considered one of the most affluent towns in the area. They have a really good rec. program, and their HS students have been going to Salem for a number of years, but starting 2009, will be going to their own HS.
Hope this helps

I do think the the public water/public sewer issue may very much help you to make your decision!!