Given that you'll normally have the engine running when using the in-car entertainment equipment I can't see the battery making any difference.
Maybe the battery on your previous car was faulty anyway?
Kevin
you`re kidding, right ?

If it will physically fit, go for the bigger battery !!

Ah, are we talking Macca Dee's car park?

In that case, perhaps a separate battery would be the best solution because it can be located closer to the amps it's powering and isolated from the battery you're relying on to start the engine, so if you over-indulge you won't be treated to the shame of a jump start.
Another 15 Ah isn't going to do that much for you if you're already draining a 70Ah battery, though, IMHO.
I have a feeling someone tried to fit the 85Ah battery listed for the 3.2, to a 3.2, recently and it was far too big suggesting that that line in the trade club book is a misprint. My 3.2 certainly has a 65Ah battery and I can't see anything significantly (physically) larger fitting without some surgery.
I'd urge you to measure up before parting with any cash.
Kevin