The processor speed numbers are a little deceptive. Each generation of the MBP comes with a newer generation processor that performs better than the previous generation at lower rated speeds. Additionally, overall performance is not just a factor of the processor in play but also the speed of the RAM and storage.
So the Retina MBP, for example, comes with SSD storage that for the 2012 model is around 500 MB/s read, write.. the 2013 model bumped that up to 800 MB/s read, write, where the latest generation rMBP's are up to 2.5 GB/s read, write..
The latest generation forgoes a lot of the different connectors for all USB-C ports which require dongles, but the 2015 and previous models don't..
So..perhaps you can tell us what your budget is and what kinds of things the laptop would be used for. The use of the laptop will go a long way in letting us know to recommend a particular variant over another..