London/Paris or Paris/London Fall 2018

Either way is workable and the biggest consideration are your flights - book into one city and out of the other so you don't have to backtrack.
Assuming you haven't booked your flights yet, I'd recommend flying into London and doing the London leg first. For first timers, I think London is a bit easier since there is no language barrier. If you want to include Stonehenge and Bath, I'd suggest spending first day after arriving in London to acclimate. Then the following day you could take the train to Salisbury and then take the bus to Stonehenge - if memory serves, there is a "hop on Hop off bus that runs hourly. Spend the night in Salisbury and then take the train to Bath the following day and then train back to London.

I would do London first because of the reasons Nancy lists. Delta has some direct flights to both London and Paris from Detroit.

ALSO, I suggest you give yourselves at least one extra night in Paris before the tour starts. Do 2 if you can. There can be problems on the Eurostar or in fact any mode of transportation and it's just better to give yourselves some flex time. Last time I took Eurostar to Paris it was 101 minutes late due to a cable theft along the line. I was meeting up with an RS tour but had given myself an extra night so all was well. I hate to miss out on the first meet-up!

I've done the Paris tour and wow, did it ever go a LONG way to making me feel comfortable in Paris! I can't get enough of it!

If time/budget are not an issue, you might also consider doing the Paris and Heart of France tour along with the London tour.

We did the Paris and London tours back-to-back in 2012, in that order. We flew into CDG and out of Heathrow. I was a little concerned that it might be better to start with London first because there would be less of a language barrier. But, they speak so much English in Paris it was a non-issue.

Either way is perfectly fine.

Just remember a couple of things.

Always a plus if you can arrive a minimum of a day before and stay a day or two after each tour.

Book the Eurostar in advance. 3 months if possible. Cheaper fares.

Fly open jaw. Into one city and and out of the other. No need to back track unless you wish to.

If you do fly to London first (and you'll get varying opinions on this as far as taking the Heathrow Express vs riding the tube or taking a bus) book the Heathrow Express tickets in advance as well. Cheaper.

My argument for the taking the Heathrow Express is based on 3 things:

1. I stay near Paddington Station.
2. Comfort and ease
3. I don't dawdle.

After taking the red eye from LAX I want to get to my accommodation ASAP and start my vacation.

RS tours are great! You are seeing 2 wonderful cities for your first European adventure. Have fun.

Flying into London and out of Paris is often cheaper than the reverse, due to the departure taxes from London. But you have to check. Go to Kayak or Google Flights and select Multi City (not One Way or Round Trip), then search for your dates to see what routes and prices come up.

Thanks to all for your suggestions. We have decided to fly into London a couple of days prior to the tour to see Stonehenge and Bath and the fly out of Paris. Many of your suggestions have been most helpful. Since we usually vacation in the Caribbean and Mexico this will be a new adventure. I suppose it also means no swim up bar, beach palapa or pool side drink service in 2018.