Thanks for your interest in serving our country! To your first question, my personal experience was to attend college first, and then join the Marines as an officer...admittedly, that plan was formed because a) I was going to attend college no matter what, and b) I was lucky enough to get a ROTC scholarship. But several of my college friends joined the Marines as officers without participating in ROTC - they just wanted to serve and knew military experience as a leader is a great way to start a professional career.
As far as whether to enlist first or to go college, either path is excellent. My personal opinion is that attending college first provides more options on how you want to serve in the military - specifically, whether to serve in the officer ranks or the enlisted ranks. To be an officer a college degree is required...so if you have that done, you can choose either the officer route or enlisted. Without the college degree, your only option is to enlist - so I say give yourself more options with the college degree under your belt. You can also try to do both - enlist in the Reserves while attending college full-time. That way, when college is complete you'll not only have the options I referred to but you'll also have real military experience under your belt. The risk there is that if your Reserve Unit gets called to Active status, you'll need to put your college studies on hold.
To your last question, the only difference Active or Reserve will have on your opportunities in the workforce is based on the length of time and what you did during that time. Hope this helps!