Maybe someone can else can chime in on that. I'm not very good at finding case law, but if I have time I might try to find it.
In addition, some cops like to "play games" if you assert your rights. "Oh so you wanna play hardball?" "This would be a lot easier if you just comply with my requests, please step out of the car". Google what a Terry Stop is. Sometimes you will assert your rights and they will take you outside of the car and sit you down and question you with all kinds of questions. "If you tell me where the dope is I'll go easy on you" "Just tell me who is selling dope and I'll get you a good deal" "If you tell me the truth now it'll be a lot better than if I catch you lying", etc. Basically they can and will try to intimidate you to get you to change your story or confess under pressure and this will take longer. Just remember that they can't have you out there forever, only a reasonable amount of time. And also remember you can choose to be silent at any time. All you have to do is identify yourself to the officer and provide any vehicle paperwork he requests. Beyond that, you pretty much don't need to say a single word except to decline a search.
Usually it doesn't work well that way. If he tells you to stay put and you get up that could be bad. I have been yelled at before by a pissed off officer because I took the time to close my door and lock it. The police can lie, they can be pissed off, and they can yell at you for something that is completely legal and within your rights to do. Like I said, they may intimidate you into not doing something that you have every right to because you don't know what will make them mad enough to do something like handcuff you or arrest you. You should try to close the door as you're exiting the vehicle to make things easier. If you make any move while exiting the vehicle or getting back up to close the door that could be perceived as you hiding something, that could end up being very bad.
The meanings of the words reasonable and probable pretty much tell you the difference. Reasonable suspicion is what any reasonable person would believe given what they know. Would a reasonable person have a suspicion that you had done a drug deal if they knew you had been arrested for drugs before and they saw you exchange a handshake and conversation with somebody? Probable cause is a higher level of suspicion. Is it probable that you have drugs in the car? If it smells like weed, yes. If there's a pill container in visible sight, yes. If there are needles in the car, yes. Nothing in sight and no smells = no probable cause. A dog alerting that there are drugs in the car is probable cause. The officer you encountered was operating on reasonable suspicion to initiate contact with you. He had no probable cause and you didn't admit to anything so he could not search your car. If he had probable cause, he wouldn't have even asked you if he could search your car, he would've told you to get out and he would've tore it up right in front of you. Just remember reasonable suspicion is a lower level of proof and probable cause is a higher level. If you are being asked consent for a search, they don't have probable cause that you've done anything illegal so hold your ground and don't give in or admit to anything. Remain silent if you must.
Now would be a good time to visit
http://www.flexyourrights.com and see some examples of enforcing your rights.