Depends what you're using it for. I have a 2.8, and even with a stuffed injector pump/injectors, i can still spin 33's fine with a stock motor. I love it, considering how i've driven it since i've owned it it's held up fine. Alot of other members on here love their 2.8 and can apparently spin 35's, and with chip/exhaust and a little more boost a 2.8 should pull all the bar work and gear just fine.
3.0l are a bit quicker, but have a tendancy to go bang, apparently. But in my opinion so does every other motor if you treat it wrong. Whichever motor you choose make sure it has a good service history, and receipts for any work that the seller claims to have done.
For heavy towing though, i'd go a 4.2 if you can afford it. All round a more reliable motor, and from what i've read on here, cheaper and easier to modify.