In my opinion, itIt is best to do minification server side for a few different reasons
- Future proofs in the event you change CDNs. Minification will always be on your side and you won't need to worry about that feature if you change CDNs.
- Ease of troubleshooting. If there are any issues with the minification you can control it on the server without having to involved CDN support.
- CDNs are designed, and architected, to deliver content, not necessarily optimize the content.
Also, I'd recommend only a single CDN provider. Multiple CDNs can over complicate things. Look at CloudFlare and their RocketLoader product. I'm not sure about price, though could be useful.