A few things you want to consider:
(a) When you do a heterogeneous environment like this, you're going to be a little limited in terms of doing automatic upgrades. The detection servers that are NOT on the same platform as your Enforce server will need to be manually upgraded when you do an upgrade. Not that big of a deal, but something to consider, especially if you are deploying lots of detection servers.
(b) Features and function should be the same whether on Linux or Windows, EXCEPT with Network Discover. DLP uses Windows drivers to reset the last accessed date/time on files when it scans them. Therefore, Linux Discover servers are not able to reset the last accessed date/time when scanning. This may come into play if you have backups being done based on those timestamps, etc. There may be some other nuances with regards to certain types of targets that should be considered. All things considered, I'd probably opt for Windows servers for Network Discover if at all possible.
Regards,
~Keith