I am literally 50-50 as to whether to go for the internship in China or to do a CELTA course in Vietnam.

Can anyone convince me either way?

I like the look of the social side of the internship, and being with 100 like minded people for the first two weeks...but is a celta qualification better in the long run, as well getting a better return than the monthly allowance given on the internship?

Any advice will be very gratefully recieved as I need to make a decision on this asap and am caught in two minds!!

An online TEFL certificate and a CELTA/Trinity CertTESOL are two completely different beasts. The former is perfect if you just want to teach for a year or so, and you don't mind where or for whom. The latter two are the most respected and accepted entry-level TEFL qualifications on the market. If you are serious about having a career in TEFL or you want to teach in competitive locations such as Western Europe, they are THE way to go. And if you want to teach in a native English speaking country, they are essential.

Aside from cost, the big difference between the CELTA/Trinity CertTESOL and online TEFL certificates is the 6hrs' observed teaching practice. Without this, none of the 'big name' schools will even consider hiring you. Ever. However there are plenty of schools all over the world who are happy to hire you regardless of which qualification you have, and many users of Chalkboard are testament to this.

Only you can make the decision since only you know what you want from this experience...