Ben - As you know, I've been spending four months in Scotland for a few summers now... always WITHOUT a vehicle during my stay! Don't worry about using public transportation to get around. Public transportation is plentiful, well organized, and extremely affordable. If you use trains, buses and taxis you will come out much cheaper than renting a car, although it's considerably less convenient. That's the tradeoff... there is less hassle renting than using public transport. Only you can decide which you prefer, but for an American, using public transportation is a bit of an adventure.
If you decide to rent a car, check if one of your credit cards offers rental insurance for bookings paid for exclusively on the card. American Express offers this, as does most VISA cards. I've never paid for the supplemental insurance in the UK when renting a car. The cards require you to refuse the "excess" coverage and pay for everything on their card. They then cover you if there are any issues during your trip. You can find details and instructions on the card's website, or you can call customer service from the number on the back of your card for advice. I've had a couple of fender benders in the past and the credit card coverage has always been excellent and smoothly handled.
Hope this helps.
Whit