Polyurethane doesn't build well. It doesn't adhere well to itself. Multiple layers require sanding between layers to increase mechanical adhesion. It's difficult to put down more than 1/64th thick at a time. Plus, poly has to dry before it can cure. The thicker you build it up the longer it will take to cure and harden. Finally, poly us 50% solids, so thick layers will shrink and can crack.
Since poly is only 50% solids you would need twice as much poly vs epoxy for the same thickness. That makes the two equal in cost... because epoxy is 100% solids. Epoxy can be done in two or three thicker layers. Both poly and epoxy will cost about $3 per square ft for a 1/8th inch thickness.
I disagree about several durability and ease of use things you stated above (see my avatar for my poly bar). You have some good points but I can speak from experience that I built a bar top with old maple stain poly and put on 40 coats. Yes I know, crazy.... But I was just out of school and I didn't have the money for the pour on (have had the bar for over 10 years through 6-7 moves). I still have that bar and the finish is great and has held up to lots of use. You can slide mugs across it cheers style.
Poly is VERY durable for this application and should seal and become water proof in 3 coats. If you want to build the poly up use 000 Steel wool to scuff it up between coats so it adheres properly. You should be fine with 3-5 coats tops though, not my 40.
My process was, lightly scuff with teh 000 steel wool, wipe off steel wool filings with a rag soaked in mineral spirits, apply poly. Let the poly dry where it isn't tacky between coats.
You would have to do some thing about the post cards though, like laminate them or something. And I am not sure that you would ever get a smooth top with the cards under the poly. However it can make a killer bartop on a budget.
All that said you should use the pour on epoxy if you want to put something like coins beer caps or photos underneath. You will never be able to build up poly the wya you can the epoxy. Since it isn't self leveling you will never have a smooth surface where your photos or coins are (every coat will add thickness to everything). You would have to build it way up then sand it down level. PITA
If you want to make a waterproof durable bartop Poly is a great tool - even if you only use 3-5 coats
I built a queen ane styled coffee table at the same time out of ash and applied 4 coats of poly. That surface has stood up to the test of time as well.