Both are owned by Graham Wylie and have similar records over fences, with last year's Albert Bartlett runner-up Boston Bob scraping home in both starts.
Back In Focus has shown more in three outings, culminating in a narrow defeat of stablemate Aupcharlie in a Leopardstown Grade One over Christmas.
Of Boston Bob, Mullins said: "Pedigree suggests a longer trip.
"Back In Focus stays all day. He has the same owner as Boston Bob and they won't go for the same race.
"Back In Focus could go for either the four-miler or the RSA. "Boston Bob is in the RSA, he could drop back to the Jewson but we will see nearer the day.
"We will try to pick the easiest option for both."
Elsewhere, Flemenstar's owner Stephen Curran said his horse did not show his true colours in defeat against Sir Des Champs at Leopardstown on Saturday.
Flemenstar was held up by jockey Andrew Lynch and was beaten by a length and three-quarters in the Hennessy Gold Cup, but Curran wants to see if something was wrong with the gelding before important decisions are made.
He said: "He seems okay, but he underperformed. He wasn't himself. Andrew said he had to push him into the jumps, which he never has had to before.
"He didn't carry Andrew at all and we cannot put a finger on it, there was no life in him. He didn't sparkle.
"We'll have him tested over the next few days and maybe we will find he had got an infection, or there was something amiss."
Flemenstar is entered for the Gold Cup, Ryanair and Queen Mother Champion at the Festival.