Hi Mark,
Welcome to IIS.
If you upgrade to a 10" you will probably have to upgrade the mount aswell and preferably to a EQ mount as they track the stars better.
I'm just a beginner too so I wont give any advice on what is best to get but the 2 most important things I think are.
1. portablilaty If its too big and heavy you possably wont use it as often as a smaller setup
2. The mount is the most important part. A good sturdy mount will give better results and allow longer exp. times.
Maybe look at getting a EQ mount and sticking the 6" on that at see how it goes, and if your sill not happy or get apature fever(its a very common problem, and there isn't much of a cure for it)
I'm sure that others will jump in and give some better advice as to what to get.
If you can tell them what you are taking pics of then they will be better be able to point you in the right direction.
Just my thoughts.
Cheers and again