Ljubljana is a beautiful but small city. There is not necessarily much to see, but it is a vibrant city with a lot of restaurants, stores, etc., and it is beautiful to walk around especially at night. It makes a good base for day trips.
Lake Bled is a resort town. The lake is beautiful but not that big. You can walk around the lake in about an hour, visit the island, see the castle...it might not take you that long. The town of Bled did not impress me - I stayed over there a few nights to do exploring (with a car) of the region including Lake Bohinj. Some might enjoy staying over in Bled to relax, for the vacation atmosphere or just being near a beautiful lake, but my own preference would be staying in Ljubljana but day tripping to Lake Bled.
I drove to Bled with my own rental car, but I believe a bus is considered the most efficient way to visit the lake by public transportation. You can also take trains, but I think you can't get directly as close to the lake by train as you can with the bus.
To see how to get from town to town in Europe, try the website www.rome2rio.com . For train connections for Europe, try the website www.bahn.com .