As long as your soil doesn't contain too much lime, I'd go for this one
Susan for me too. Always covered in flowers and from an early age too.
Susan's nice, mine died, but daughter in Cormwall has a nice one, as mentioned, it flowers from the start, almost, some you wait for years to get a flower.
i have this one, http://www.waitrosegarden.com/plants/_/magnolia--soulangeana/classid.4146/?gclid=Cj0KEQiAlO20BRCcieCSncPlqqMBEiQAOZGMnKtW86Zf4I4F_G_i9M7bvmEvF8MsXmen59a-37nU348aAuaN8P8HAQ&gclsrc=aw.ds a present from Lanhydrock, still only about 4' tall at th moment but did flower last year. Also a Stellata in a big pot.
Not so long ago I went round and counted our Magnolias. I found 13 so we had to go out and buy another one. Then I found one I had missed,so now we have 15.
Buy one in flower. We got one labelled Wadas's Memory which is a lovely white. Ours flowered purple!. Too late to return it even if we could remember where we bought it.
Not sure if Susan will reach 12-15ft except under optimal conditions, around here [N.W England] most older specimens are around 8-10 ft in height. A variety with large purple flowers which reaches 15 ft is Iolanthe. If you like star magnolia types then stellata has a big brother called Magnolia x loebneri which slowly gets to 15 ft. It has a pinky purple variety called Leonard Messel. Both these varieties usually flower about 3 years after planting and are hardy in the climate in north west England.