21/18 is for sure the way to go. 21/15 is a solid build, but you'll find you're having to shove your shield on your back / wear extremely light armour in order to attempt to keep up with the 2H sword AGIspammers (most people...). And advice would be, avoid them, but as mentioned -- they are the bulk of melee people. And you won't be able to out run them, you can't spend your whole time avoiding the bulk of people -- it's just not very viable. Adding more than 21 strength is just gluttony as a 1h shielder, the extra HP / hitting power is nice, but you're more than capable of killing people with 21 strength and you'd be better served keeping in range and getting a speed bonus for your hits with more AGI.
21/18 allows you to keep up with them in armour that isn't paper thin, it also allows you to dodge around a lot more effectively when being gang banged by 2 - 8 people causing them to hit each other and you get free hits on the back people. 21/15 just doesn't give you the manoeuvrability / armour / acceleration to pull that off consistently. with 21/18 you can spring surprise forward attacks on people that want to keep you on the edge of their massive range advantage.
I also think not wearing paper thin armour is a big plus, you don't have to wear super heavy equipment, but medium tier body armour, decent gloves and a helmet that allows you the ability to still move fast (don't bother with legs). The mantra that your shield is your armour does not hold true in a group gang bang where you're going to catch a few hits occasionally no matter how good your footwork is. It's not possible to always avoid such a situation in Battle. It's nice to not die in 1 - 2 hits, which can mean you surviving til the end of the round and killing 1 - 5 more people.