Remember that the measure of C is related to the battery capacity. ie: A 2500 mah battery 10C would mean 25 Amps, while a 4000 mah battery 10C would mean 40 Amps. So using your numbers, the TP2500 20C would mean it can handle a 50 Amps max (Burst) current, and the TP4000 18C would handle a 72 amps current. I hope that makes it easier for you to choose.

Regarding the 5000 mah vs 4200, I think you are missing the flighttime parameter. The 5000 should give you quite more flighttime. But I don't know the weight difference. The TP4200 is heavy enough, and where I fly it gives us about 12 minutes of flight time with a power 25 motor and a 13x8 APC prop. I prefer to fly mine with a lighter battery, as it performs better on the air... I use TP2100 batteries with a Hacker A30 10XL, and have flight time of about 5 minutes while staying on the safe side. I've used the TP4200 on my plane, but it feels quite heavier. Anyhow, I'd never change to a heavier battery but my friends who use the TP4200 would neither trade theirs too, so I guess you should give it a try.

Regards,

Javier