The only pro to milling a slide to accept one type of optic is that it's almost always the cheapest option.
The downside is that you're married to that foot print until you buy another slide.
A slide milled to accept adapter plates is THE only way to go if that route is available. I've been down both roads so I'm speaking from first hand experience.
Slides milled for adapter plate systems are mostly an OEM feature. If you have a CZ or a Glock, there is an after market solution for an adapter plate system.
CZ Custom does it for CZs, and Agency Arms does it for Glocks:
http://www.agencyarms.com/agency-optic-system-aos-cut
If I were you I'd send it to Agency Arms for their AOS system and be done.