@Jonathan B. hard to say in general, much of the area is street-by-street. The streets numbers run south to north in Union City: so 1st St is the southern border of UC and Jersey City, 49th St is the northern border with UC and West New York.
If you care about proximity to NYC, I'd recommend nearer to 30th St. (closer to the Lincoln Tunnel). Being north of 30th St is also advantageous: it's where more of the businesses are, where city hall is, and closer to some shopping areas (Pathmark grocery store is at 40th or so) and also Weehawken. Buses to NYC run down Bergenline (Bergenline runs in one direction through Union City, north to south) more frequently than buses run to NYC run on JFK or Palisade Ave (in my opinion). 42nd, 43rd, 44th etc. are seen as very nice, and I also like the area around the Monastery (18th-21st St near Bergenline). Bergenline is sort of the main commercial drag of Union City, although there are also shops/restaurants on Summit and Central.
Streets below 10th St are a little farther from New York and there are fewer shops and restaurants and more factories (auto repair, sausage factor, etc.). You are, however, closer to JC Heights and, if you're farther east in Union City (closer to New York City) in that area you have really easy walking access to Hoboken.
Anywhere on the eastern border of UC (Palisade or Park) is expensive, though usually quite nice, because (some) of those buildings have views of Manhattan and are right on the bus line.
It's really very much block by block. Some blocks look bad (burned out buildings, loud tenants, etc.), but you can turn the corner and you'll have beautiful, well-maintained brownstones.