If you click 'make keys on own server' on CertSimple you'll have two options: ECC or RSA, with ECC being the default:
ECC certs are a newer technology that is smaller and faster than equivalent RSA certs. You should always use an ECC cert when possible:
That's why CertSimple defaults to ECC. However there's a few things some users should know...
Nearly every current web server, CDN or cloud provider supports ECC, however there are a couple of notable exceptions - we expect these to change shortly, so check the links below to ensure they're still accurate.
If you use:
Then you'll want to use an RSA key.
Additionally, out of date Mac OS machines that run OS X 10.9, which can't handle ECDSA/SHA384 signatures, will have trouble connecting to ECC certificates. The current version of Mac OS is 10.12, any release newer than 10.9 will work properly.
See who doesn't (yet) support ECC HTTPS certificates for more information.