Having owned a boxter for 2 years, and then having switching to supras, I can only say that its a balance of what you want the car to be/do. The boxter is an incredibly stable car but underpowered. Country roads are a bliss to travel through and no corner really proves to difficult for it. Track days in a boxter are fantastic fun, but you end up wanting more power once you've settled into it - and that costs mega bucks.
Mine had far too many problems with reliability in 2 years for me to justify the price/running costs, so I went for a supra.
The supra is a very competant car all round, but it is bigger and heavier than the boxter, so don't expect the same dynamics - that said the grin you get everytime you push the right pedal far outways the difference. They're not as agile round country roads, but track side they are unbelievable. Once you've had some track time, you want a little more power - so you upgrade bits n pieces to give you more power and you get back on it !
These are my opinions on the two cars - the supra wins my heart everytime - I've never regreted changing - so much so that I'm on my second supra - I wouldn't have bought another boxter......