With this trade-in you are most likely gonna be spending more in the long run. Seeing as you would have had to buy the 3DS beforehand - which is about £120, now I think. Then after trading it in, you spend an extra £150 on the XL, pushing the amount of money spent around £270. And it goes even higher if you got the 3DS for the ridiculous launch price.
atm cex give you £69 cash for a nintendo 3ds boxed which is nothing, i paid £200 for it when it first launched so i feel quite sick if i have to sell the thing for the 3ds xl but that's life you get fudged over all the time, especially from game who offer you 10% cash for your goods and then sell it at 90% higher at a profit second hand, like you said it all adds up.
i think by the time i sell this 3ds to get the xl it will be worthless unless i sell it to friends or family.
I know where you're coming from with this, i got my fingers burnt too with purchasing the 3DS at launch.
It's taught me a lesson at buying a console at launch, i know Amazon has the 3DS XL at 179.99, but whether it will remain at that price only time will tell.