4u and 3u jjs will be both be better in power department than mx70. Obviously 3u will have more, but will be less manoeuvrable as well.
It's only a difference of 5 - 10 grams. Maybe you will see a 5% difference with the 3u hitting harder but every other shot will be easier with the 4u. Considering you were using a 4u mx70 I would stick with a 4u jjs because the jjs is more head heavy no matter what you do.
Victor x Transformers Optimus Prime & Bumblebee Limited Edition Badminton Rcaquets and Bags are NOW for PRE-ORDER If you are a fans of Transformers especially the Autobots..then now is the chance you dont want to miss...After the success crossover with Iron Man last year, this year Victor has come back with TRANSFORMERS.....This time they only interested in the 2 main and most popular character out of Transformers which are the OPTIMUS PRIME and BUMBLEE BEE.http://racquetforce.blogspot.sg/2014/05/victor-x-transformers-optimus-prime.html
For those in Singapore who want to get the transformer racket bag. I remembered texting this person 8128 2935 He was selling some racket bag in buy/sell section. So u can ask his price. Just sharing.