Crentist is correct in saying that if you configure both POP3 mail clients to NOT delete messages on the mail server that you could run both Windows and Mac clients using POP3, however there are a couple of issues with doing this.
Firstly, each time you fire up mail client in each OS it will have to download the email again. Depending on how much email you get this could mean downloading a lot of mail unnecessarily. IMAP has the advantage that the mail doesnt travel down your net connection unless you actually retrieve the message, typically the mail client will only download message headers in advance. FYI, both Mail.app and Outlook can handle IMAP connetions, though whether your email service does is obviously something you would need to investigate.
The second issue is that each PC you need to download mail on would need to be individually setup and you MUST ensure that this setting is set. If it is not and you set off Outlook for example and it downloads and deletes messages in error, your email would no longer be visibile in Mail.app when you went back to it.
POP mail clients work out what they need to download each time they connect by looking at the server and comparing with the local cache of previously downloaded emails so it does not depend on MAC addresses or any such strategy.
Having made the switch to GoogleMail some time ago I would ( and do ) strongly recommend this as an approach. Googles SPAM filtering is simply the best I have seen. In the last couple of months I have only had 3 items of junk mail make it through to my mailbox. This is compared with previously when I was using Mail.app and its built in junk mail filtering where every day I had around 30-50 items of junk mail manage to get through.
This and the fact that I can review my email anywhere ( even on my Blackberry ) make Googlemail the clear winner for me. I actually use Google Apps for Domains so my muxton.com emails are delivered directly to Google.
The other aspect is that Googles mail service is infinitely more reliable than my ISP's service which regularly used to have long mail delays due to problems with the service. Mail sent to Google arrives virtually instantly.