Both John Gosden-trained horses won at Royal Ascot, with Coronet taking the Ribblesdale Stakes, while Stradivarius landed the Queen's Vase. The colt seemed to carry the better credentials, but the prospect of easy ground meant Dettori went with the filly instead.
"She's strong, she's been racing in Group Ones for most of the year and I'm pleased to be on her," Dettori told BBC Radio 5 Live.
"Stradivarius has won over two miles, but all his best form is on top of the ground. There are showers forecast and it's not going to really dry out, so Coronet was my obvious choice.
"It was very hard, though, and I left it until the last minute. Half the field can win it, probably even more. It's a good St Leger, all the good stayers are in it."
The spare ride on Stradivarius went to James Doyle, who won the Gold Cup on Big Orange.
He told ITV Racing. "It's nice to get a ride first and foremost, especially for Mr Gosden on a horse who has improved all year.
"Frankie made his choice based on the ground, so I'm hoping the rain stays away.
"It's further than a mile-six, he's shown a bit of pace at Ascot and had tactical speed in the straight. Obviously he has the stamina as he beat Big Orange in a Goodwood Cup - that's not easy as I well know.
"Watching all his replays, he's a beautiful mover, so we just hope the rain stays off."
Champion jockey Jim Crowley aims to cap a stellar season with a first Classic success when he teams up with Crystal Ocean.
Two of his most significant victories this year have been on Ulysses in the Coral-Eclipse and Juddmonte International and it is for that horse's trainer Sir Michael Stoute that Crowley has a great chance of glory in the world's oldest Classic.
"I got a phone call from Sir Michael asking if I'd like to ride. Obviously, it's a great ride to pick up and I'm looking forward to it," Crowley said.
"He was probably a little unlucky at Ascot, but he looked very good last time in the Gordon Stakes and it looks like he's improving."
Standing in Crystal Ocean's way is the mighty battalion from the Aidan O'Brien stable, with Irish Derby hero Capri leading his four-horse assault.
"Capri is a horse we always thought would stay further than a mile and a half and he's in good form," the Ballydoyle trainer told At The Races.
"He had a break after the Irish Derby. We were playing with the idea of running him in the Voltigeur, but we knew that he would have a 5lb penalty and his scope wasn't 100 per cent, so we just decided to leave him.
"The Irish Derby was a very truly-run race. He was handy all the way, he kicked and kept going. I don't think he was relenting at the line."
O'Brien also runs Venice Beach, Douglas Macarthur and The Anvil.
Andrea Atzeni is looking for a third St Leger, after wins on Kingston Hill and Simple Verse, with Roger Varian's Defoe, who is unbeaten this season.
"Defoe has proved a revelation this year, winning all four of his starts, and I love the way he has taken his racing, improving considerably with each run," Varian, who trained Kingston Hill, told his website.
"He has shown the necessary speed to win over 10 furlongs, but he relished the step up in trip at Hamilton and Newbury, and the form of the former race has worked out brilliantly, while he cruised past some high-class horses in the Geoffrey Freer in really taking fashion.
"He is quite versatile in terms of ground, he just doesn't want it quick, which it won't be, while the extra furlong promises to suit him as well.
"He is in great order and his homework suggests he has stepped forward again - I can't wait to run him in what looks a high-quality renewal of the race and I think he will produce a mighty performance."
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