Barry Geraghty admits "it's a tricky one" as he ponders between More Of That and Cause Of Causes as his mount in the Randox Health Grand National at Aintree on Saturday.
JP McManus' retained number one rider, who only returned from injury on Sunday, is likely to delay his decision until as late as possible.
Though McManus could also represented by the Charlie Longsdon-trained Pendra and Anthony Honeyball's Regal Encore in the big race, Geraghty has narrowed down his choice to two.
The Jonjo O'Neill-trained More Of That was sixth in the Cheltenham Gold Cup, while Gordon Elliott's Cause Of Causes won the Glenfarclas Cross Country Chase at the Festival.
"I'm none the wiser at the minute. I've sat on both horses." Geraghty told At The Races.
"I rode Cause Of Causes work last week and he felt well. I schooled More Of That on Friday and that went well, too. Both are in good shape.
"You can make a very good case for both horses, so I'm a little bit undecided at the minute.
"More Of That was running well at Leopardstown when he can down at the last. You have to think he disappointed a little in the Gold Cup, but you'd like to think he's better than that.
"Cause Of Causes obviously won the cross-country race in Cheltenham and that has been a good indicator over the years, but off his handicap mark it could be hard for him.
"It's a tricky one. It's not a straightforward one.
"I'll make my decision on Thursday or shortly before. I can't imagine it being too long before Thursday."
Aidan Coleman and Jamie Codd are among the riders McManus can also call upon.
"Aidan Coleman will ride one and possibly Jamie Codd," said the owner's racing manager Frank Berry.
"If the ground stays like it is, Regal Encore will probably run as well.
"At the minute we could have four runners."
Robbie Dunne is looking forward to teaming up with Gas Line Boy for trainer Ian Williams.
"He has some good form in the book. The last day he was fourth in the National Trial at Haydock," said Dunne.
"I rode him in a piece of work and he seems to be going very well."
Brian Ellison is hoping Definitly Red will get that important slice of luck in his bid to land the great prize.
The North Yorkshire handler said: "Basically, he's going there in great form, and he's schooled well.
"Last year I would have agreed that he might not stay the trip. I doubted it last year, but this year he's stronger and better.
"When he won the Grimthorpe Chase at Doncaster, he went on four out and he wasn't stopping.
"The ground will help him, good ground will help him.
"He's been awesome over the National-style fences. If ever you've seen a horse over a fence, he was good.
"If he gets the luck, he's got a great chance."
The McManus-owned Carlingford Lough was amongst the defections at the five-day confirmation stage as 70 horses stood their ground.
His absence has left last year's runner-up, the Kim Bailey-trained The Last Samuri, at the head of the weights, which have gone up 1lb.
Carlingford Lough was one of six horses who were previously in the top 40 ruled out on Monday.
Alan King's Ziga Boy was a general 50-1 shot, having won the last two renewals of the Sky Bet Chase at Doncaster, but he is set for a lengthy spell on the sidelines after suffering a tendon injury.
Twelve-year-old Maggio, who carries the same colours as 2013 National hero Auroras Encore, also has a problem and while it is not so serious, it is nevertheless a big blow for his connections.
Other horses taken out include Alechi Inois, Foxrock and The Romford Pele, leaving La Vaticane and Doctor Harper jointly in 40th spot, with the official elimination order yet to be confirmed.