Skip the Harland, not worth the extra overwintering effort it will require. It is also not as good for bonsai as Kingsville, or any of the smaller leaved varieties of Buxus Microphylla (Japanese boxwood).
Life expectancy is about 300th on the list of reasons to get a tree. Not important. Actual age is not important either. It's about 200th on the list (unless you're working with actual ancient stock, like over 100 years).
What is important is hardiness and ability to LOOK old. Kingsville has many factors that make it an excellent choice for bonsai--tiny leaves and good "believable" trunks, even on smaller trees. It naturally grows tight leaves that can be a pain to keep in line, as they get dense, but lend themselves to very nice foliage silhouttes.
Kingsville and other tiny leafed varieties of B. Microphylla (Wintergreen, compacta, Koreana, and others ) tend to be extremely winter hardy and tough. They work into pretty nice little (and even tiny) bonsai. I've had Kingsville and Koreana cultivars for decades. They can work into extremely shallow pots. If you're looking at stock plants, get the one with the largest trunk you can find. Kingsvilles are extremely slow growing, so bigger is better--and probably more expensive). I picked this one up 15 years ago from a woman in MD who has stock from the nursery that originated the variety. It had been field grown for 20 years or so from a cutting. Worked it into shape and into that pot within three years of getting it.
