I am aspiring to eventually go into galaxy imaging, which would mean I would need a longer focal length for such an endeavor. For this, I am currently aiming to get a newtonian. The types of newtonian I am considering are 6" f/5 and 8" f/4, which would give me 750mm and 800mm focal lengths. I would most likely be pairing one of these with a 1" sensor astrocam. The reason why am I not going for a bigger scope is that I live in a very light-polluted city, and require long trips to rural areas for imaging, which means portability is a big factor for me.
I am very much tempted to get a 8" f/4, due to its slightly longer focal length, and even more so due to the greater speed of the telescope. However, compared to the 6" f/5, I would like to know if it is much more difficult to collimate an f/4 compared to an f/5? Would an f/4 be able to hold collimation just as well? Are there any other factors I should consider in this respect?