most resin are very good mate in my opinion, but for your question mb400 is better than dowex ..for me....... and for one reason, when its spent, it goes to 003 004 005 ect the dowex goes to 000 to 050 straight away, just shoots straight up...
i can get mb400 for £59.00 plus vat and £5.50 delivery... inbox me for orders
hope that helps
I think you'll find it's the other way round!
Purolite MB400 is recognised by almost every DI user these days as the worst resin you can buy! When spent, it's spent!
Dowex MB50 is very much better than the above
Tulsion MB115 however is better than them both.
This has a bigger impact on twin DI systems, if you're using RO's & not a heavy resin user then it won't make a massive difference to you to be honest.
Nearly 7 years as a DI only user & I have actually tried & tested them against each other over prolonged periods of time.

Regards
Winp