Best focal ratio for amateur-sized Newtonians is f/7.
Seems to me a lot of people are buying faster ones without realising the effects of coma, astigmatism and field curvature will severely limit the useful field of view and, with what you'll spend on extra bits of (expensive) glass to try to alleviate these, they'd be better off if they had opted for a slightly longer scope in the first place.
If compact tube length really matters then buy an SCT.