From previous experience, either configuration will support the volume of hosts and checks you've described. So other factors need to be considered -- like cost, supportability and ease of administration. From an installation perspective, I find that the majority of Nagios users seem to prefer Linux-based (and Red Hat is amongst the most popular) installations.
This Nagios User Profiles page shows some statistics on this and indicates that 30% of users who registered use Nagios on Red Hat and only 4.6% use Nagios on Solaris. From a support perspective, knowledge and responses on the mailing list and other forums do tend to be skewed toward Linux-based installations. As such, I'd recommend the Linux-based approach. Good luck with your installation.
This was first published in June 2007