Here is my 2c (Take it or leave it kind of thing).
This is my personal experience, and there is a 99% chance that none of this will apply to anyone here.
I consider myself a Ford guy (I started Fordmods), and I have worked on a lot of Fords, and I currently own a VE CalaisV V8.
(So I dont consider myself biased for or against either brand).
My father currently owns a G6E (Non Turbo), and has owned Fords most of his adult life (EA, EF, BA).
Firstly, the BA XR6T, is it a Manual or Auto ?
(Reason why I am asking is that the BTR / ION 4 speed auto boxes are terrible, the VE V8s get the 6 speed Auto boxes, which means its easier to keep the engine in its sweet spot when shifting).
If you have the manual 6 speed, then this goes out the window of course.
Now comparing the FG G6E Turbo to the VE CalaisV V8, here is what I found,
Pros for the FG..
1) The Interior (For me anyway) was much more ergonomic and comfortable than the CalaisV. This is still the case after 2 years of CalaisV ownership.
Things like the centre and door arm rest position, seating position, rear vision, are much more comfortable than the sportier inclined VE.
(Hey I own a CalaisV, I do expect some comfort !)
2) The ZF 6 speed in the FG is much better than the GM 6 Speed Auto. Much smoother, and does not hunt around gears at low speeds as much as the VE.
3) The I6 Turbo is cheaper to mod than the VE V8s. (Im a big fan of the I6, and will be very sad to see it go in the next couple of years).
Here are the cons against the FG,
1) Turbo Requires premium fuel (Not a big deal, but it does add up over time when the VE happily sips E10, or any crap that it is fed without farting, pinging, and carrying on)
2) Insurance (The FG G6E Turbo was twice as much to insure for me than the VE CalaisV V8) To give you an idea, I have declared $5000 worth of upgrades to my insurance company, and I am currently paying just under $800 a year.
3) Lack of workshop manual. (Please PLEASE someone prove me wrong here and show me WHERE I can obtain an FG workshop manual, either as a subscription service like the GM eSI website, or purchase as a DVD, hard copy, in fact ANYTHING !!!!)
This was a HUGE setback for me, as I like to work on my car myself, and without a workshop manual for these things, its impossible to do anything fun !)
4) Parts are cheaper for VEs when compared to FGs. (Examples include brake upgrades, panel and interior parts, and upgrades such as HSV bits and pieces).
Because there are a LOT more VEs out there when compared to FGs (This is not a reflection of one brand being better than the other, just stating sales figures), used parts are cheap, and consequently because of turnover, new parts are cheap also.
5) The VE CalaisV gets a lot more features than the G6E. (The G6E is a kinda inbetween Calais and CalaisV).
Extra features found on the CalaisV that are not found on the G6E include,
projector headlights, rear seat entertainment system (roof mounted DVD player), 8 way electric driver and passenger seats, rain sensing wipers, front and rear park assist (audio and visual), heated mirrors are the things that I can name off the top of my head.
In the end, it comes down to what you want the car for, how you are going to drive it, and what mods do you intend on making when you get the thing.
To be perfectly honest, you cant go wrong with either model. It just comes down to what is the better fit for what you intend to do with the car.
I hope this helps, and also gets you thinking.