The end of the finale of Torchwood Season 1 goes directly into the third episode from the end of season 3 of Doctor Who. You won't be completely spoiled if you watch Torchwood season 2, but there definitely are a few things that come up in season 2 that have to do with season 3 of doctor who.

Thanks, that's exactly the kind of information I was hoping for! So "End of Days" into "Sound of Drums"? Hmm, not sure I have the restraint to wait that long, but Sidereel lists a six-month difference between their airdates, anyway, so I gather I won't be losing any essential storytelling tension by finishing Torchwood 1 than waiting to catch up on Who. Guess it'll just be down to whether my urge to catch up on both series as quickly as possible is stronger than my conviction that's it's unnatural and possibly blasphemous to watch Doctor Who anywhere other than late-night PBS.

So "End of Days" into "Sound of Drums"? End of Days into Utopia, followed by Sound of Drums and Last of The Time Lords.

There's a bit of a link-up between DW S3 and S4 at the same time as the point between TW S1 and S2. Then, there is a meet-up at the end of DW S4 as well, which takes place after TW S2 ends.