But does Namecoins serve the same purpose better?
Bitcoin seems to be more active, and seems to attract more computing power, AFAICS.
And Bitcoin network cannot be very accurate for this because it can vary block times for +/- 2 hours right?
On average, every 10 minutes a new block is generated. So yes, that timestamp could be 10 minutes after you sent the transaction, or even some more minutes later. In addition, the block is only "really" assured after 6 transactions, so add 1 hour just to be safe.
However, I doubt that this will cause any problem, because it's usually not about minutes but days. I bet that quite some time elapsed between your discovery and the creation of the dataset whose SHA-256 hash you published. So I guess that adding 1 more hour (maybe 2) won't make any difference.