Answer: Rate of growth really depends upon the growing conditions. For your purposes, I'd eliminate 'Variegata' and concentrate on your other two choices:
Buxus sempervirens 'Fastigiata', in 20 years, will reach a mature height of 10 to 12 feet, with a spread of 4 to 5 feet. Best grown in average, medium wet, well-drained soil in full sun to part shade. Prefers sun-dappled part shade and moist, sandy loams with a slightly acidic to slightly alkaline pH. Tolerant of pruning and shearing. Avoid cultivating around plants because they have shallow roots. Roots appreciate a good mulch (1-2 "). Thin plants and remove dead/damaged branches annually to improve air circulation. In USDA Zones 5 and 6, this boxwood is best sited in a sheltered location which protects it in winter from strong winds and full sun. Carefully remove heavy snow accumulations as quickly as practicable to minimize stem/branch damage.
Buxus sempervirens Monrue (Boxwood Green Tower) Reaches 9 feet tall, but is never more than 2 feet wide. The habit of this plant is very dense, yet it keeps its neat, straight-up-and-down shape, as if you'd just finished shearing it. In fact, you never have to do a thing to the foliage, but if you want to shape it, Green Tower is dense enough to create topiary without leaving unsightly gaps and holes.
The foliage is medium- to fine-textured, giving Green Tower a full, solid look even from a young age. Expect it to reach about 5 feet tall within 5 years (9' tall in 10 years) in partial shade.
Buxus sempervirens 'Variegata' is a dense round evergreen shrub. Very slow growing to maybe 5' in 20 years. The variegation changes with the season, from white to cream to yellow.
Q&A Library Searching Tips