AFAIK, the smaller the kernel - the better
It depends on your configuration.
Smaller kernel = more modules = increased time needed to boot, more flexibility (module loading without need to reboot); Bigger kernel = less modules = less hard disk I/O = faster boot, but less flexibility (no module loading/unloading on the fly);
So it depends on what you want and what is your hard disk capable of. On laptops (4200rpm) compiling a monolithic kernel can shorten boot times, because kernel image itself is compressed, thus resulting in less HD I/O. On SATA raid there is almost no decrease in boot time when loading modules at booting, thus you can compile everything as modules (well almost everything...) and have a very modular, small kernel ready for hotplugging your PCI cards.
In linux it is all about choices.