Well,you could always do both. Or do pre meal some days and post meal other days.
In a way, they're telling you different things. So it depends what you're wanting to know. Testing pre meals will show your blood levels at their very best, and give you an idea of how well your control is going. Testing after meals will show you whether that meal was a good one for you or not, whether it contained more carbs than your body was able to handle, resulting in an unacceptably large spike (although its not unusual for bloods to be a little higher two hours after eating). To accurately judge how large a spike is, of course, you need to have measured before and after. However, after a while when your more familiar with your body, you may find you don't always need to bother with the before test - I can tell if a food has spiked me by just looking at the after test, because my before tests are always pretty much the same.
However, as a type 1, that may be different because I would have thought you needed to know your before figures to know how much insulin to take? Or are you on a fixed regime of insulin?
Type 2 on Metformin, diagnosed Jan 2013, ultra low carber, Hba1C at diagnosis 8% (64), average BS now between 5 and 6 mmol.