The STR 447 tube is a Sovtek made tube.....can be kinda harsh and brittle in some amps..think of it a a utility type tube...gets the job done. If you have a dark sounding amp...this tube is a perfect fit, but it's definately EL34 sounding!
the STR 450 is a new old stock (NOS) Seimens made tube. Definately not as brittle...warmer and smoother responses. I have a pair of these tubes in my personal MK IV along with a pair of 5881 tubes and get everything that I need....the clean sounds similar to a Fender Bandmaster, the crunch similar to a Hiwatt and lead pure boogie..smooth with no jagged edges.
Of course, there is a big difference in cost, but you get what you pay for.