My wife and I will be in London the first week of May. I think it would be nice to take one of the Hop on Hop off bus tours. They both come with a river tour.
Which one is best-Original or Big Bus?

6 of 1, half dozen of the other. Different fodorites will recommend one - some the other.

One little known suggestion -- If you board at one of the stops where both companies have competing agents (barkers really) selling tickets on the sidewalk - they will often get in a bidding war for your business. You can sometimes get your tickets for several £ less than face value. Doesn't happen all the time -- but I have taken the tours many times (usually I'll get first time visiting friends settled in on the bus and then hop off somewhere along the route and meet up w/ them later) and probably 1 out of 3 times we get a discount.

Tussauds group may operate one or the other as they were advertising discount tickets to Madame Tussaud's Wax Museum with preferred entry if you had a ticket for one bus line or the other. so if going there you may save a bit with one or the other.

otherwise after scrutinizing these two i agree with Janis about being peas in pods

Tussauds also operates the Eye Over London big wheel

I guess I will go with the one that is cheeper.

Look at their websites and make sure your choice stops at places you want to see. Their routes are pretty similar, though.

We used the Big Bus in May and had a good time. We only got off at the Tower of London - our heads were starting to nod a bit - but it was fun, interesting, and it was easy to get the next bus. We booked online and got a discount.

Lee Ann