The nice thing is that Vail is close enough to Breck to do both. In fact, you can ski Keystone, Breck, Copper, Vail, and Beaver Creek -- they all are within 40 miles of one another.
Breck has lots of green runs -- and lots of the transitions are pretty flat. It is also at 9500 feet at the base. So, if you are coming from sea level, you may have problems acclimatizing to the elevation. Beaver Creek and Vail have base elevations around 8,500 feet, so you are less likely to have issues with the elevation. But they are generally warmer, so if your visit is in late March, the snow cover may be less complete. But if it is a warm spell, you may also have the hardpack warm to corn earlier or more completely than at the higher elevation resorts.
Vail has a great deal of terrain -- much of it accessible to skiers of the ability you report. But Vail and Beaver Creek are much more expensive than the others -- I spent $20 for a burger, fries, a soda, and a candy bar on Saturday. The same meal at Copper on Sunday was $14 (no bargain, but still cheaper than Vail).
So, tradeoffs. You could also consider Aspen. Snowmass has a great deal of terrain, and may be less crowded because of its remoteness to the Front Range.