That would depend on what you are actually doing.
The very basic GPs should be similar, just that GPflow relies on tensorflow for the gradients (if used) and probably some technical implementation differences.
For the other more advanced models, both libraries provide references to the respective papers in the docs. In my opinion, GPflow's design is mainly centered around the SVGP framework from [1] and [2] (and many other extensions.. I can really recommend [2] if you are interested in the theory).
But they still do provide some other implementations.
I use GPflow since it works on the GPU and offers a lot of state-of-the-art implementations. However, the disadvantage would be that it is under a lot of change.
If you want to use classic GPs and are not too concerned with performance or very up-to-date methods I'd say GPy should be sufficient and the more stable variant.
[1] Hensman, James, Alexander Matthews, and Zoubin Ghahramani. "Scalable variational Gaussian process classification." (2015).
[2] Matthews, Alexander Graeme de Garis. Scalable Gaussian process inference using variational methods. Diss. University of Cambridge, 2017.