It depends what you're wanting to do with your trimmer. For light intricate trimming and edging of maintained lawn areas for relatively short periods a loop handle is the way to go. If you are going to be trimming open areas for long periods or want to use a blade the bike handlebar is the way to go. Because you need to use a strap or harness with a bike handlebar unit, reach is limited...you can't one hand them down slopes and such. It's also very cumbersome to turn them on their side to edge with. They're cumbersome in general really but the strap/harness makes it easier to use them for long periods without tiring. I use my loop handled units 95% of the time.
P.S. - Your dad would probably be more comfortable with the bike handlebar unit with a harness.