We will be arriving in London 2/15 through 2/23 and we want to go to Paris for two nights.I'm having trouble deciding how best to do this. Our flight arrives Heathrow at 9:30 am. Should we stay in London that night since we'll be dead tired or go straight to Paris?
If we stay in London do we check into our flat and then check out the next day carrying all our luggage? And check back in 3 days later?
Also, if we go straight on to Paris would it be better to just stay at Heathrow and catch a flight over or go into the city and take the Eurostar. I'm not sure how much time to allow for a flight change with customs. Would 3:00 be resonable?
I appreciate any advice. Thanks.

I'd head straight for Paris, to minimize the unpacking and repacking. You can get off a flight at 9:30 and be at Waterloo by noon (by Underground). Catch a few winks on the train and take in as much Paris as you can (some of the major sights stay open late in summer) before you bed down.

Well - I would have done an open jaw ticket. But it seems to be too late for that now.

I would NOT hassle w/ trying to get from LHR to Waterloo. I would fly from LHR to Paris as soon as possible after landing and stay in Paris for two nights (get over your jet lag there)

Then take the Eurostar from Paris and spend the rest of your trip in London.

I also think that flying from Heathrow is a good idea -- you save some time packing and unpacking also by heading straight to Paris. Check BMI, BA and Air France for flights.

Is an open-jaw not possible?

Keep in mind that getting on a flight out of Heathrow is infinitely more hassle than jumping on the Tube if you have to go through Customs & Immigration and then Security again. If you can stay in the Transit Lounge, you're okay on that score, but it will still cost you a bundle to fly. For the cost of the one-way airfare, you could get a 2-day first class Britrail Days Out Pass + Eurostar round-trip. If you fly, you'll pay for airport transfer in Paris plus the cost of the train to London.

If you decide on Eurostar, I just got a $90 round trip (available on trains leaving Mon-thur only between 11am and 3pm). It's not all that hard getting from Heathrow to Waterloo. From the airport take the air bus to nearby Feltham station (about a mile from terminal 4)and catch direct trains twice an hour right to Waterloo, where Eurostar leaves from. In Paris you're in the city center rather than at CDG - a hour and at least $15 by train out, or if you take a cheapie flight maybe to Beauvais, much farther out. these tickets are hard to get, only a few at that price on trains - my agent had to search several trains to find one. But if you act far enough in advance - can book up to 120 days in advance and tickets are non-changeable/non-refundable, go far it. I buy through Budget Europe (800-441-9413) and they are extremely helpful! The next cheapest fare is $90 one way, and again only on Mon-thur. Weekends cost much more. These cheap fares are available in London and Paris but problematic getting one without advance reservation i think.

I don't think it will cost a bundle to fly. This summer, I got two one-way tickets on BMI for LHR-CDG for a total of 60 pounds inclusive of all taxes and fees. I'd be more concerned about the two-ticket issue, since there is a possibility of delay for the flight coming into LHR. Check-in and security were only a minor hassle.

Open jaw isn't possible as I bought the tickets rdtrp. PHL to LHR weeks ago and then decided to go to Paris afterwards. I have so far only checked BA and one way is £336! I am afraid of the time issue with the possibility of a delayed flight and customs, that's why I thought the Eurostar might be more feasible.
I'm thinking it would be quicker to take the underground into London and then a cab to Waterloo and be on the Eurostar quicker than re-checking in at Heathrow and reticketing luggage. Am I crazy?

Why not check BMI - http://www.flybmi.com ? Their fares for LHR-CDG are usually much cheaper than BA.

Yes, definitely give BMI a try.

If you want to get to CDG try and do it without having to clear customs etc...check BMI, I agree....much much easier.

Okay, I checked BMI and it's £8.99 for a one way. Definitely cheaper. But I still have to clear customs coming off the plane from Philly, correct? They have a 12:25pm flight and a 3:25pm flight. Will I be able to make the 12:25pm since I'm arriving at 9:30am or should I book 3:25pm just to be safe?

After my last experience at Heathrow, where we had to wait 85 minutes inside the plane before we walked down the steps onto the tarmac and onto shuttle buses and then to Terminal 3, I would expect delays for your London-bound flight.