My 18th birthday present was an experience day like your one (although that was a decade ago now.

and obviously the Gallardo wasnt around then ).
Anyway I opted for the single seater racing, and I think I made the right choice. The the drivers who picked supercar or rally days had to have the instructor sit with them the whole time, making sure that they didn't go into corners too fast or push too hard. Wheras on the single seater day they
obviously can't accompany you. So after a bit of classroom tuition and a few laps in a mid range BMW (or similar) provided you show that youre not gonna be a liability your left to your own devices around the track, (bar a few pesky track marshalls of course).
The rev needles on the single seaters stick at the highest point you take it to, and are checked at the end of each of your sessions, so un/fortunately? you cant go mad out there! (we were told to keep below 4k IIRC) It is still however bags of fun and I guarentee you'll come home smiling

and wanting to go back for more.
Whatever you decide to do im sure you'll have a great day.
Btw. Which track is your voucher for? Mine was Oulton Park and the back sections of that track are so much fun.