5K iMac or Mac Pro?

I'm having difficulty choosing between these two. Obviously the costs are different but the choice would be between a 5k iMac with 256 GB Flash (and I'll manually upgrade to 16 or 32 GB ram) or a refurbished Base model Mac Pro. The reason I'm having this conflict is that I already have an excellent high resolution (although not quite 5k) monitor and I'm worried my current ultrawide monitor (34 inches!) along with an iMac may appear cramped on my desk. On the other hand, although I need a fairly powerful computer I have a feeling that a Mac Pro might be an overkill. Thoughts?