Some of the Uberti guns have some glitches. I think theyve upped their game in the past decade or so, but Ive seen several older reports of barrels that didnt index in and torque correctly and were loc-tited in, that was probably 15 or so years ago. Screws are often mentioned as very soft, enough so that theres aftermarket sources for complete replacement screw sets for many Uberti guns, and the tab on the back of the loading gate, often called a ladle by many today, that can be reinforced or possibly replaced with an aftermarket part, i dont recall the details. So, small things that can be addressed, like screws and loading gate. Google "1873 loading gate" and you should find info on the cowboy action forums about it.
The Miroku I believe is better made in general, no screw issues, possibly the same issue with loading gate, again, its been a couple years, and i think can be modified to help it. Not enough difference to be earthshaking, I just prefer Miroku guns if its a choice and practical.
If I wanted a specific model type and a Miroku wasnt available or was insanely expensive Id get a Uberti, probably in the Cimarron name. They are generally felt to be a little better finished and they have cooler barrel markings than standard Ubertis. I want an 1866 carbine at some point, Miroku didnt make any carbines, Uberti/Cimarron is the logical choice.
I will just say if you decide to get a Miroku 73 and find an example for sale in your budget, absolutely do not hesitate or it will likely be gone pretty quickly. They made only limited numbers of most models and there may not be any more.
One personal gripe, Uberti mounts the carbine front sights on the barrel band. It was done by Winchester for a short time then they got smart and mounted a stud silver brazed to the barrel with a blade pinned it it. I hugely dislike the sight on the band, but it doesnt seem to bother others. I had a 22 so made and it was a big pain in the arse if it got bumped and the zero changed. Id look into getting a proper sight stud mounted if i had an Uberti carbine. You might get by with drilling the band and barrel for a pin to help keep it indexed. Smith barrel pins or Ruger rear sight pins may work for that. If its a range toy it may not matter. My stuff goes out in the hills, I want to know its on zero if it gets dropped or falls over when leaned against a rock or tree and the dog gets rambunctious or whatever.