I guess it depends on how long you want to harness for. If you want to harness for as long as possible (the child may be unable to sit will in a booster for many years or you just feel more comfortable in a harness), then the regent will probably be your best bet as it will allow harnessing for probably a 1-1.5 years longer then the frontier.
My child is off the growth charts on height (she has a long torso too), and at 6 years old, I keep on questioning how long she will last in the regent. The straps are right at her shoulders so if she has a growth spurt, she is out of the seat for good. The regent is outgrown when the harness falls below the child's shoulders or the child exceeds the weight limit for the harness, or the child's ears extend above the top of the carseat.
What is your child's torso height? How much does the child weigh? What seat is the child in right now? Where do the child's shoulders fall in relation to the slots on that seat? If the child is long torsed, then he may not get a lot of time in the frontier harnessed, but at 5 years old, the child may be very mature and be able to sit well in a booster seat. I don't like to see 3-4 year olds in boosters full time, but at 5 and above, a booster seat can often be a valid choice (provided the child sits properly in a booster seat).
You may want to find a store that has the regent and frontier in stock so you can check how well it fits your child and in your car. Both seats have installation issues in some cars so it is best to see how well they fit prior to buying them.