Seraph' made an excellent point, Do what you can excel in normaly the one you enjoy the most!
You need to be comfortable so it can become second nature to you. Trying to defend you self when you have to think = less time you have to react and more time for your aggressor to make a move.
Daxe' explained it pretty well about the different forms of martial arts Don't just limit your self to picking between the 2 there are many others! Attacks normally start while you are standing and end up on the ground. So both Grappling and Striking are needed for defense.
Pepito' is right about Finding the right Instructor, Sensei, Coach or w/e, You will find alot of bad classes out there believe it or not, Some are just in it for the money, are you watching? Poof they just waveed 20% and give you a Free T w/ a Gym Bag if you sign up for a year "because you will quit in 2 weeks". . . Some don't allow Full contact if any contact at all. . . Some are sickos, the list goes on. . .
Wuntvor' was saying about Having some Friends or Family attend the sessions with you. To make it a little more fun while at class and out side the class. If You want the master A technique. Once, even twice a week of just classroom exercises isn’t enough imo.
Its all a matter of preference tho. I’ve tried them both and am still active in Judo & Jujitsu, Id rather do Aikido instead of judo but cannot find the classes :-(. As for Karate, its just not my style, partly because I didn’t like my sensei lol, That and Take down and ground arts just seem to click better for me. I do it for a sport, to compete, have fun and stay in shape. Self defense is just an added benefit.
What is better for self Defense??? IMO just avoid the situation or retreat be for a fight happens and you shouldn’t ever need to fight x-D.. .but yea. . . There is no what is the better style. Only who has the better skill level and the upper hand in the situation.
Getting in Shape??? Ground martial seem to be more of a work out , you will find yourself using more muscles + the Wight of an opponent. where the arts of Striking is less exhausting, but can be a more fatal sport. When it all comes down, It just depends on the level of work out you are given and not what style you are doing.
I’m Sure if you go down to the gym and ask the Instructor if you can watch or even join in for the day He/She will be more then happy to let you in that days class free of charge. When you think you finely found the right Instructor, Martial arts and Type of work that's offered. DO NOT rush in to it, Always sigh up for the minimum normally a month to see if its right for you. That way you can try Karate one month then Judo The next and compare what ones you enjoy more if any. Even when your Month is up sign up 1 Mo at a time. I hope this Helps ^_^.
P.S. IMO Grappling martial arts "Judo" would be better then Striking martial arts"Karate". But we all know Striking martial arts just look alot more cooler:-D!