Welcome to GMAT Club, Sun!
I'm a fan of MGMAT's approach, but everybody learns differently, so it's awfully tough to say whether Manhattan GMAT
's courses will work well for you. My first instinct is that a 420 is a really low starting point for their classes, and you might feel a little bit lost at first. Manhattan's target market is people who are chasing elite MBAs, and most of their students are starting at a reasonably high level... so the course might move a little bit quickly for you, at least in the beginning.
That doesn't necessarily mean that you'd be better off with a GMAT tutor, though. I have nothing against GMAT tutors, but most of the good ones are pretty expensive, and if you're going for a 280-point score improvement, you might want to do everything you can with cheaper options (self-study and classroom courses) before you hire a tutor. You might benefit from a customized approach from the very start, but you'll probably end up spending a huge pile of cash if your goal is to improve by such a huge margin.
Before you start the MGMAT course, you might want to spend some serious time working on some of the basics, especially on the quant side. MGMAT's Foundations of GMAT Math
book might be a good place to start, so that you at least have some of the fundamentals down before you start the class. And if you decide to do even more self-study before starting the MGMAT course, there are plenty of resources and ideas available on this lovely little website.
Good luck with your studies, and please eat some of that amazing SF dim sum for me.
Helping students kick the GMAT in the nuts since 2002... www.gmatninja.com.