Well, there isn't a "better way" With CPM you pay for brand awareness, and with PPC you pay for traffic/conversion. So it depends what is your product or service.
It depends what you are targeting. If you need to get people on your website and conversions- clicks, for brand awareness-impressions
It's all about your CTR (Click through rate), if you know that your CTR is high then CPM might be better for you but usually PPC is better as you can measure everything, do tests, improve your campign, get accurate numbers etc...
As other users have mentioned it depends on what you're trying to achieve. Normally when you're running a campaign for Brand Awareness one uses CPM. On the other hand when one is after performance (ie. making a campaign profitable) CPC is the preferred choice normally.
It all depends on your goals, what you exactly want to achieve. There are three ways to run paid campaign, CPA, CPC and CPM. If I am looking for only sales then would go with CPA, otherwise with CPC and CPM and If I am just looking for visibility then ofcourse would go with CPM.