I owned a Scarabeo 150 before my 400. With the 400, I get better fuel economy (honest!) on regular instead of premium gas, carry _tons_ more stuff, feel a lot safer, can get parts faster (though the service still isn't perfect), don't have to route my trips to avoid the interstate anymore, and, best of all, can check the oil without removing any plastic. (I'm told on some 150's this is not necessary. On mine, however, it was. It took a good five minutes.)
The 150 was a great bike to learn on; don't get me wrong. And I hope you're happy on yours for a good long time to come. But, when the time comes to go with a Burgman, I predict the experience will, after a day or two's worth of adaptation, be one of unmitigated joy. The only time I miss my 150 is in parking lots, and then not very much.