I haven't played PG, but it looks almost exactly the same, but older. UoC is certainly worth buying, though.
I got the demo and just played the tutorial, half a scenario and skimmed through the manual. I will be playing a bit more later this week, but so far I like PG more. PG just seems to have more depth, except for the supply lines stuff, which might get interesting. Things I dislike compared to PG (but keep in mind this is only at first glance):
- Basically you only have two types of units in UoC: infantry and tanks (in PG you have AA, artillery, recon, fighter planes, bombers (tactical & strategic), tanks, anti-tank, infantry, subs, destroyers, aircraft carriers, ...). I especially miss my planes and arty.
- Terrain seems to play less of a role, although some types of terrain favor the defender. In PG your infantry reigns supreme in e.g. cities, forest, hilly terrain, while tanks reign in open spaces. Therefore you have to really look at moving your units around in such a way as to optimize their deployment (e.g. move your tanks in front of the infantry in open terrain, have your infantry clear any forests you have to cross first, ..).
- There seems to be less depth in attacking and defending. In PG AA and arty gives automatic cover to any unit in front/above it that gets attacked, anti-tank is better at defending than attacking.
- UoC seems to be about keeping your supply lines intact and encircling enemy units, while PG seems to be more about combining your forces in exactly the right manner, for example crossing open terrain with infantry behind your tanks, then position your infantry in front of an enemy city, while your arty gets positioned nearby to provide cover, and your tanks circle around the city to take out enemy AA and/or arty. In the next turn you then first weaken enemy entrenchment with arty and perhaps some strategic bombing and then attack the city, while you tanks move on to the next target.
- PG seems to be more fast-paced, but perhaps that's just the era they've chosen (early WW2 blitzkrieg vs mid WW2)