Platinum support main here - it really depends on the game your playing. You should be looking at all sorts of variables before deciding on your boots. As for damage vs defense, that really depends on you and your playstyle. **Building defensive items is never a bad idea. **
Here's a quick and dirty rundown on when you should / should not buy each set of boots:
Damage Enhancement for AD / Auto attack based champions. You buy these to enhance your damage output. It's a BAD idea to buy these if the enemy team has a boatload of CC and lockdown.
Mobi boots are the most versatile boots in the game. They can help you chase or poke, play to lane pressure, and roam for ganks. If you lack the ability to STICK to your target, or your team lacks control, these can be a BAD idea. **Most players will underestimate the chase potential of these boots. **
Essentially the best movement speed boots in the game for in-combat scenarios and extended group teamfights. It's almost never a BAD idea to buy these boots, unless your looking for something else. I think these get overlooked more than they deserve. **They are especially potent against a set of champions with POWERFUL slowing effects like Zilean and Nasus. **
Probably the trickiest set of boots. Especially potent with the new Rune System, there are some busted combinations out there that can seriously alter the time frame of your summoner spells. I feel that these get overlooked in favor of the other ones being more desirable.
Mercies are the prime choice against AP and CC focused champions. Almost always a good choice, unless they are playing mostly AD focused. Most players will buy these if the enemy team has a very fed AP user. BAD idea if they are rolling an almost full AD team.
Tabi are essentially the opposite of Mercies. You want to buy these if they have a lot of AD potential or a fed marksman. BAD idea to buy these if they have an AP-centric team comp.
Last but not least, these essentially increase your damage on AP style champions. These are very common, and more damage is always good. It's a BAD idea to buy these if you are getting targeted and feel unsafe in teamfight style scenarios. This really depends on the enemy comp and how easy it is for them to catch you.