>>> " . . . both seem to be about the same thing on offer"
It's probably the same holiday!
A quote from Door2tour.com's website:
"The Website is a marketing website only and Door2Tour.com does not supply Tours, products or services and acts as an agent for Operators featured on the Website".
As there are only a fairly small number of coach firms in the UK who will be operating tours to Switzerland at Christmas, it's quite possible that the tour you're seeing on the Door2tour.com website is actually the one that's part of Leger's operations.
Looking at this tour, as an example, I see that the reference number given for it is D2TLEGMAF:
https://www.door2tour.com/tours/leger/picturesque-lake-constance-feldkirch--zrich-christmas-markets-maf
The 'LEG' bit within it strongly suggests to me that it's actually operated by Leger.
My own instinct would be to cut out the middle man (Door2tour.com) and book
directly with the tour operator (Leger Holidays). You'll then be handing over your money to a company that's fully ABTA bonded, rather than to one that isn't.