It really depends on what you're after. Proving the benefits of a twin setup compared to a single and vice versa is a long debate and I'm already tired of it. It all has to do with what kind of powerband you desire to get of your motor and how you want that power to be delivered.
If you want low end power and torque accompanied by linear, more useable power delivery and still be capable to make decent power then the twins would be better than a very good, late ball bearing GT turbo. On the other hand if utmost power is your goal and you don't mind a bit of lag (any hesitation below 2-2.2k RPM is considered lag by my measures :evil2: ) then obviously a big single is the way to go and for less money, that's for sure.
Talking about money though I think that if you go for a decent single setup the difference is not as much as most people think. A new Garrett GT3540R which is one of the best turbos you can match to the 7mgte with a nice power curve and 600hp potentials is about $1600-1700. I got both my T3s (fully rebuilt, ported and polished) for $660. Then you could go with the OBX manifold that is readily available on ebay for about $450 and spend about $300-400 for a custom twin downpipe. Then I'd say another $100-150 would go for the custom two-in-one hot side i/c pipe and you double the cost of whatever air filter you'd use for a single. Finally another $100-150 for your extra oil piping and that's it really.
So basically the total cost for the twins is about $200-300 more than just the price of a good single turbo that gives a comparable performance. And I haven't calculated any expenses for the GT3540R's custom oil lines and it's hot i/c pipework.
Now if you do the research and you are aware of the benefits of each setup, when you come to incline to the twin setup and you really want to go that way then $200-300 won't stop you. Don't get me wrong, I agree with everyone here that such a set up is a real pita and it gives you more headaches than usual but if linear response and balance between big power and control is what you're after then the twins will deliver.
My only objection in some ppl's posts is the "with the single you'll be better off on the long run" comment. Could someone explain what you mean by that? I can't see how two turbos will be less reliable than one if they all get treated/maintained exactly the same way.
And if the photo of that crazy piped monster scared you there, here's what my engine bay will look like when I'm done, with the exception that my car is actually LHD in contrast to the RHD in this photo:
