Hi Sammy,
I have also hiked both trails a number of times and totally concur with Dave's information and recommendations. If you do the Highland Trail, do not miss the Lookout by Starling Lake as it is quite spectacular. Make sure you go the very end of the Lookout Trail where you are perched high atop the rocks. If you're good with the challenge aspect, then the Highland Trail should be feasible in the 3 day/2 night timeframe.
I guess the big questions would be how much hiking/outdoor experience you have, what is your physical condition, and how heavy will your pack be? Also, how much solitude are you craving? I made the mistake of going a bit too light and too far for my first solo trip which made things a little less enjoyable, but served as a great learning experience! If you're looking for a larger distance challenge, I suppose you could consider loops 1 and 2 of the WUT, but I'm not sure if it's a great idea for a first trip, especially solo.
Another option is the third loop of the WUT by Rain Lake. It is less busy (relatively speaking!) and you have a shortcut/bailout option from the Otterpaw/Gervais Lake areas vs. doing the whole loop if need be. The out and back section to the top of the loop mostly follows an old rail bed and is easy to traverse, which is kind of nice to start and end a hiking experience. The loop itself has some good challenges and some nice spots along the way. I would avoid the Ishkuday Lake site at all costs! I can offer further recommendations if interested...
The Killarney loop is amazing. We've done it twice - once in 6 days/5 nights and once in 4 days/3nights. We're in pretty good physical condition, but the 4 days was a bit much! Also, you'll need to book this well in advance as sites are in high demand.
Hope this helps - cheers!
Rich