summerbird OPKs and temping look at different things IYSWIM. Warning, am going to get all scientific now....
OPKs detect the hormone surge that prompts ovulation, there are 2 dark lines if you are fertile, no line or faint line means you aren't. They are helpful when they work, only sometimes they don't, even when you are ovulating. And the decent quality ones cost around £16 per month, so it's not cheap! Also if you have irregular cycles, you won't know when to start/stop using the sticks.
Charting BBT works on the principle that progesterone, which is released after ovulation, causes your body temperature to rise. When you see a sustained rise, you can be fairly sure you've ovulated. It won't tell you when you're about to ovulate though - when you see the temp rise the egg is almost certainly gone (or making a baby!). Charting is useful to (a) check you're ovulating (b) to get to know your cycle, so you can predict when you might be fertile in future cycles (c) so you know when to expect your period. It also saves on pg tests because if you're not pg your temp drops just before your period; if you are, it will stay high or for many women it goes even higher.
It is cheaper than OPKs - costs £10 - £15 for a BBT thermometer (you need one that goes to 2 decimal places, Boots do them)and nothing thereafter. You can register at FertilityFriend.com for free, enter in your data and it will help you work out when you've ovulated.
Down sides are remembering to take your temp before you get out of bed, some people find that a faff; also it is quite addictive and you can end up reading far too much into the data (like I did last month!).
I can really recommend the book 'Taking Charge of Your Fertility' by Toni Welscher, it's long but well written and easy to read.
Phew, I went on a bit there didn't I
All this has become my 'Mastermind specialist subject' over the last few months