We will be staying in the Bloomsbury area close to the Russell Square underground station. I am wondering if we should take the Piccadilly line or hire a car service to Heathrow. We would need to depart our hotel about 7:30 am on a Wednesday in October. The two of us will each have a small suitcase. Thank you for your advice!

Considering it's rush hour, if you're willing to stand for the first part of the trip, then the tube is fine. I choose my accommodations near the Piccadilly line for that reason. Obviously a car service would be nicer, but not necessary.

One more thing, having used a car service once, the car is more prone to getting stuck in traffic and so for that reason the tube has an advantage. I'm sure you'll hear from the usual suspects here who have experience with more than one trip in morning traffic so you can make a more informed decision. But road traffic varies so much between 1 trip and another, for that reason I now opt for the tube.

With a small suitcase the tube will be fine.

Leave earlier to avoid more of a rush hour and possible delays in Tube service - 6:30 am would be more comfy I would think - especially going out of London as MdePerdu says after central London. The perils of any car service getting caught in London's horrendous (oft) traffic should be a factor.

We have an international flight departing out of Terminal 5 at 11:30. I was thinking leaving at 7:30 was plenty of time to arrive at the airport 3 hours or so before departure. I'd like to take advantage of the hotel breakfast, if possible. Is this cutting it too close?

No not at all - I was thinking of dodging rush hour on The Tube but you could even leave later than 7:30 - 8:30 and IME escape some of the rush hour travel.

>>8:30 and IME escape some of the rush hour travel.<<

Not if the airline wants you ate the airport 3 hours ahead. What do they say? Plus it wouldn't avoid crowds since the tube will be much more crowded at 08:30 than at 07:30.

07:30 will be fine.