The alba is much more fragrant than the champaca. The champaca is easier to grow though and can be grown from seed unlike the alba. Champaca is quicker to grow too, but if I were you I wouldn't even consider the champaca, I'd get the alba, no question about it.

RP1

I have both. I am not a botanist, but a plain home gardener. I can only speak out from my experience. Both like warm weather, no harsh winter.

M alba flower is white with nice fragrance. It becomes a standard fragrant tree for lots of Asian customers. The plant has good growth habit and outputs many nice flowers for your.

M champaca flower is very popular in Tailand, creamy yellow in color. It can set seeds in the U S, so many cultivars and hybrids can be obtained. At least I have encountered two kinds of M. champaca. A few nurseries sell the plant. In SF area where it is cooler. The plant performs poorly to produce the flower. Some 6 feet tall plants hardly producing any flowers. I heard the plant is propagated from seedlings. In LA area, the plant does produce some flowers. The weather and the plant genetical composition might affect the production of the flower. On the other hand, I collect a different cultivar of M champaca. This plant has very poor growth habit. The branches are leggy and weak. However it does produce many flowers from a 3-4 feet tall plant. The most amazing aspect of this M champaca is the fragrance. Out of many times of my evaluation, it is better, more balanced and more substance than that of M alba.

Scent is very personal. I do prefer the orange which has an exotic, sensual aroma. The white smells of juicyfruit gum to me.
I've had the yellow to flower within 4 years of planting from seed. That's pushing it also with fertilizer and it helps being in the ground to get some size. They seem to like to flower from approx 7 feet and higher(meaning once the tree gets that tall-not that the flowers appear from 7 ft from the ground only).
kevin

If they are like other kinds of magnolias (some now consider Michelia absorbed into Magnolia, based on DNA and other characters; see Magnolia Society web site for history) seedlings will flower much later than vegetative propagations, may tend to be more erect and vigorous. Perhaps floral scent may vary with environmental conditions or genetic makeup of individual seedlings (as already mentioned).

Sunset 'Western Garden Book' discusses both, lists the white one as a cultivar of the other, that is as M. champaca 'Alba'.

I am very excited about some new michelias I bought for our newly landscaped yard, and would appreciate advice from you more experienced folk. I bought two champaca trees for our large front entry patio, planted in a strip along an East-facing wall. These will get full sun all morning and until about 1:00 p.m. The patio is enclosed on three sides, so this is a fairly protected spot. I worry that the trees (which are already about 5' - 6' tall) will not respond well to the pruning I intend to do to keep them clear of the eaves (about 8' tall) and to encourage lateral branching for a bushier look. I thought this would be a good location, sunny but with afternoon shade, protected from wind, with the benefit of a delicious scent at the front entry. We JUST planted them today, so if this is a terrible mistake as to location or exposure, I'd love to hear from you (while I can still do something about it).
I also bought four rather spindly scruffy Michelia Figo plants, which the dealer gave me for a song because of their condition. I have had good success with all kinds of plants, and hope I can rehabilitate these. I have two possible planting spots for these (I figured I'd keep them together in a shrubby group): along a stone wall on the south side of the house, where they would face north and get direct light pretty much all day as the sun moves along the length of the stone wall from east to west - this is on the hot side of the house, but the wall itself provides some shade. OR along a wooden fence running East to West on the opposite side of the house (the cooler side), where they would also be in full sun most of day, with a couple hours' relief from shade of large liquidambar and large oak.
Can I prune either of these plants to encourage bushiness, or would this conflict with the natural habit of the plant? Most of what I read says they are shrubs or trees - then I saw a Champaca in a wholesale tree lot that was HUGE... and I'm worried!
Thanks to you all for any advice you care to share.

My champaca is about 16-feet tall and growing. I've topped it a couple of times with no ill-effects. It gets sun from about 10-2 (something like that) and looks great; maybe that's because I have my air-conditioner overflow going straight at its roots. Works great on hot days.