Wing Chun VS Aikido
There have been some great answers already, but as someone who cross-trained in those arts and did a lot of self-defence stuff, I thought I could offer some comments.
First - it really depends on what you are after. De-escalation and running away are always the best way to preserve your skin intact. Only fight as an absolute last resort.
Second - Assuming you have to fight and want to learn about, pick an instructor from whom you can learn. Find a good instructor that will make you want to come back next week, and you will learn more than with someone who just runs a school for the money. All martial arts instructors will tell you that their art is "street useful". You do have to evaluate their claims.
Third, about the arts themselves. How long do you have to train?
If you only have a month, pick Krav Maga. I can cover the basics and pointers that will make you the most effective in a self-defence situation in such a short time... but I can't guarantee you will remember them when trouble hits!
Wing Chun, in my opinion, should be studied as a "soft" art. I find the most appealing thing about it are more of the internal aspects (form, structure, using internal rather than muscular energy) than the specific exercises.
Aikido (which I admittedly only dabbled with), take a similar approach. The advantages of these approaches is that you are making yourself move in a very efficient manner, and can rely on your whole body mass rather than just muscles. You can keep practicing - and still get better - while you are in your 80's. The disadvantage is that they take a long time to learn.
The difference between the two arts is philosophical. Aikido is aimed at not harming your opponent. You will learn how to sense their movements and react in a way that will incapacitate them without harm. Their own size and strength will not matter much. This is always useful when the police questions you afterwards.
Wing Chun is aimed at being very effective in short range and close quarters. You will learn how to apply devastating power to bring much larger opponent down quickly. This, again, is very useful in surviving tight spots.
The choice is of which is more suitable depends on your temperament, how much time you wish to spend, the school and master you learn from, the legislation and police attitude in your jurisdiction etc.
I personally find that having a sound Krav Maga knowledge is extremely useful, while learning the soft arts is the way of personal growth. I also think cross-training is always a good idea if you have the time for it.