Yes, it is true that having more AF points should help with tracking
especially when the subject is small in the frame and can be lost
between the AF points while still in the AF area. There is a lot
less chance of that happening when you have 51 of them instead of 9.
If the subject is larger in the frame though, it should be much of an
issue.
The focus re-acquire time can be a double-edged sword. In a case
like you mention where you allow the subject to leave the active AF
area and then get it back then you might still be able to catch shots
because the focus hasn’t significantly changed since you lost the
subject. However if the focus does get off track and you want to
re-acquire a long delay could mean that you will never be able to
catch it.
I am not sure if the D300 has this feature or not but on the pro
Canon cameras the user can actually adjust the re-acquire time which
seems like a pretty good option to me. The good thing on the 40D in
my experience is that the fast focus acquisition time means that even
if you do lose the focus if you can re-find the subject in the
viewfinder it is quick enough to where you can often still get good
shots after a focus loss.
Greg
--
http://www.pbase.com/dadas115/