You are right about the Tekin's ratings, but truthfully they are thought of having the same power capabilities - Castle may have derived their current rating from 6S, whilst Tekin at 4S - the current would be higher, but overall power may be the same.
In fairness, ratings mean nothing - they can be fiddled so easily, that you cannot trust any manufacturer; they will both run any motor in an 1/8th scale - that's what's important.
MMM vs RX8 debate could go on for years, so consider - is sensored required? Do I run in a very dusty environment? And other such questions, and go with what you're comfortable with - they're both great. I personally had a better experience with my MMMs, than with my RX8. Now I have an MMP - and I'm sure I'll like that too.
With the '09 spec, the drivetrain is fairly offset - so you can get away with a larger battery more easily than with a straighter-drivelined truggy. Balance will only be noticeable/particularly important on a very smooth fast track. In the UK, most tracks are a little more off-road than in the US, and it is far less noticeable.
Weight and cost of 5S and 6S vs 4S for the same battery power - there are no savings to be had as Jason rightly said. What you do gain is a little efficiency - and thus a little extra run-time. You'll need the same discharge ('C') rating for any of the voltages, assuming the total power is the same.
Frankly, I'd look at doing a battery change in the longer finals - it'll be fairly quick with a truggy - lift the side of the body up, unstrap the battery remove and replace. 20 seconds or so in all if your pit crew are well practiced, and if you have connectors like EC5s or Deans (no polarity issues to worry about). EC5s would be better with a truggy though (extra current draw).
Judging by the fact that he'll be running 20 minute finals, no-one in his area runs 1/8 electric - we are a few years behind you in the UK - 1/8th electric is still very rare, people will spend £460 on a Durango, but not a similar amount on 1/8th...
Other battery brands worth looking at include Polyquest, Thunderpower, and, if on a budget, Zippy or Turnigy's higher discharge rated packs.