The last three digits of the software release ID are the important ones e.g. in 8.0.80b ".80" indicates the major release ID where significant new features were added and the "b" is the maintenance version. This is the same numbering scheme that was used when the ICX switches were a Brocade product.
The latest version of the .80 release is 8.0.80d. The latest version of .70 is 8.0.70d. It;s just a coincidence that they are both on the d maintenance release.
Which release you choose depends on your requirements. If you want stability it is better to go for an older feature release, but if you need the latest features then the latest major release may be what you should use. In both cases go for the latest letter (maintenance) release available.
We publish a Target Path list which details the releases that are widely deployed and therefore have a high level of stability and reliability. And of course you should never deploy anything without checking the release notes first. All of the release notes and the TP document can be found on the ICX section of the Ruckus support portal at https://support.ruckuswireless.com/