If you are looking at the best return, then yes. Assuming you do a good job on project selection, you will earn much more with a CPU than a GPU per Watt of power you invest.

Of course, a GPU will get a lot more work done. This is important if you are trying to make a big discovery with your machines. The more work your machine completes, the more likely it is that you will make a big discovery and get published in an academic journal.

That is the whole gimmick of GPU's - they are much faster, but much 'dumber' than CPUs. Therefore, they are good at running simple jobs really fast, such as decoding Enigma messages.