As a frequent flier, your miles have been piling up and now you have a dilemma on your hands: should you use the miles for a dream vacation or for a seat upgrade?

Well, that depends on you and the circumstance.

Frequent flier Nancy Ilk, speaking to USA Today, usually saves her miles for a nice vacation. But decided to take a seat upgrade when she took a marathon flight from Los Angeles to Sydney.

"Stretch out ... eat, drink and enjoy the journey," says Ilk, a consultant who lives near Minneapolis. "Business class on a 747-400 ... is the best."

For Jim Sherman, a financial adviser and now a lender for senior housing and long-term care facilities, who has accumulated over 15 million miles said figuring out the best way to use miles can be as taxing as earning them to begin with.

"It really depends on the route, time of year for certain routes, and your need (or) desire to be in the front," said Sherman who lives near Boston, to USA Today.

For example, when he takes one of his frequent trips to Hawaii, he buys a seat in economy and then pays with miles and cash to move up to a premium cabin.

"Since Hawaii requires such high mileage for a full first-class ticket, I feel this is a great way to conserve miles yet fly first class on a highly desirable route," he said.

However, when flying to London during the summer, Sherman has a different approach.

"I find that it can make sense to use miles to purchase the ticket in its entirety," he stated.

There are also a few catches for seat upgrades.

"Many airlines won't allow passengers traveling abroad to use their miles to upgrade if they've paid the cheapest coach fares. Alaska and Delta have a similar restriction on their domestic flights, Gary Leff, co-founder of the frequent-flier community, MilePoint.com, told USA Today.