We live in an age of relatively inexpensive high-quality cinema cameras capable of shooting movies, commercials, documentaries, and web content. Even if you want 4K recording capability or a large sensor, there is a good selection of affordable alternatives. So why in the world would you ever want to consider renting a cinema camera when it's so easy to own one?
Camera technology is changing with lightning speed right now. Manufacturers are pushing the envelope in resolution, sensor size, frame rate, data compression, and dynamic range. This technology race is great for the industry...it means our tools are undergoing continuous improvement. However, there's a downside to this rapid change. If you invest your hard-earned money in a camera system, you need to be able to get enough return on that investment before it becomes outdated or obsolete.
If you shoot regularly...at least once or twice a week...and your jobs are all fairly similar...you can probably justify the purchase of a camera system. This is especially true if you can charge your clients a fee for the use of your camera on each job. It gets a little trickier if you can't bill them directlly for your equipment, but are rolling the cost of the camera package into the overall budget for the job. Then the camera cost becomes part of your overhead and expense for every job, and needs to be factored in when you are bidding the jobs. If you are busy enough, or have other intangible reasons for having a camera (gives you credibility, lets you be more competitive in your pricing, etc.) it may be worth the investment. But don't forget, you need to consider a lot of other factors before purchasing...not just the original purchase cost of the camera itself, but lenses, other accessories, recording media, maintenance and insurance. These items can easily add up to more than the cost of the camera itself!
If you shoot less frequently, or shoot a wide variety of job types, travel often with equipment, want to preserve your financial capital for other things, and want to use the latest technology...you should consider renting rather than buying. Your local rental facility has the expertise, the inventory and the variety of equipment that you need to do most any job, and it's all available at an affordable price. You don't need to tie up your money, spend a ton of time learning about all the esoteric accessories and add-ons for each camera system, or maintain and insure it...we take care of all those things for you!
Everyone's situation is different, but the bottom line is that renting might be the best thing for YOUR bottom line.