Quote From rewt:
A disseration is nice in that you have actually achieved something. You have written a 10,000 (or so) word piece on a specific topic and come to some conclusions. You have shown writing, critical analysis and knoweldge skills that are transferable to a lot of applications. It is also the closest thing you will do in university to real work.

Though a lot of jobs don't require it and if you want a job outside your field it doesn't matter if you did a disseratation on it. My dissertation was on something completely different to what I am doing and will never be useful outside the skills I got. Literally the only time I talked about the content was in my PhD interview. But again a dissertation is a great interview question/answer, it shows knoweldge and skills. You can use to answer stuff like "how did you deal with a difficult/hard/large task?" or use it as example for strengths/weaknesses. Also if you are equal in the job process the person with a dissertation will look better.

Basically, the advantages of doing a dissertation is you build your skills and have something to talk about in interviews. After 5 years there isn't much advantage.

PS: This is all my opinion and consider other people's advice.
PPS: You will need to write a dissertation for your Master's so doing 1 now will be a lot of help

Quote From rewt:
Not an expert but would depend on the masters course. If it is a taught masters it doesn't really matter but is nice. If it is a research masters with intention of a PhD, definitely do a dissertation.