I very very respectfully disagree with your opinion. It's all good, and it's all just opinions, but please let me explain why I see things differently.
When you leave Magic Kingdom, there are no taxis available right there. The taxis are at the Transportation & Ticket Center (TTC). In order to get from MK to the TTC, you have to take a monorail or ferry.
Okay, so now you're at the TTC. Yes, you can walk pretty much a straight line down from the monorail platform, past the ticket booths and look for a taxi to drive you to the Boardwalk area.
But since you're already at TTC anyway, I'd suggest you just walk to the other monorail platform and take the monorail to Epcot (to get from MK to Epcot you must take 2 separate monorail trains). Yes, you might have to wait for the monorail for maybe 5 minutes or so. But once you get on, it is a direct shot to Epcot. No possibility of traffic tie-ups. No possibility of other sorts of delays. But yes, once you arrive at Epcot you'll have a decent walk back to Le Cellier.
But like I said, since you'll have to get to the TTC anyway, from that spot, I'd choose monorail over taxi any day.
If this was a bus route from one park to another,. I'd certainly vote for the taxi, since they both have to drive on the surface streets and you would likely leave in a taxi before a us would even arrive to pick you up.