I've ridden both and while I COULD get along with a GT650 it's way too new to buy until some kinks are worked out.
I have my doubts the GT650 could stick with the Burgman in the twists but would top it in top end and has better suspension.
I think a Burgman 400 is more suitable to the beginning rider.
Both 650s are big powerful machines and can get you in a heap of trouble very quickly.
IF you are on the tall side and strong then the heft may not bother you.
There are many good deals out there on 650s with super low mileage ( including my own deal on an exec ) who were new riders and got themselves in over their heads.
In 45 years of riding an myriad bikes the two 650 Burgman's I've owned ( 70k km between them ) -
they are the best all around bikes I've had.
I just did the 45,000 km service on the 2009 Exec. $250 covered all fluids and service. Not one other thing wrong.
Needed a new battery after 4 years - aside from that oil changes and tires. That's it.
I doubt BMW service costs will be that low.
This is the review that hooked me on the category
http://www.onewheeldrive.net/2006/10/04/scoot-touring-the-honda-silverwing-and-suzuki-burgman/
I bought the Silverwing first but moved to a whole new world with the Burgman 650 and it's performance in the twists was a huge eye opener and the race rider in that review on VFR800 stated there was no losing the 650.
The Burgman is better than I am as a rider and I always feel it's
a) safe ( took on a deer and rider kept it upright and rode it home < former member here - I have his seat

)
b) reliable ( has always started except for one unexplained issue that seems to happen once in a very great while - we are still discussing it ) even in mid winter and after sitting months
c) it's bloody fun
I do feel the 2013 is overpriced in this economic climate and really think a 400 is a better start point for a new rider.
But for the category.....the Burgman 650 Exec is a proven performer with a lot of value and can take you grocery shopping, across the continent or blister the twisties yet so sedate and effortless in traffic.
Not many rides can cover those bases.