I will share with you my own experience.
I had that very same question during my last year of undergrad. I also asked around, saw the pro and cons of the situation, thought about it for a while and finally made a decision. my reasoning was as follows.
I love learning, i enjoy it and will do it for the rest of my life, but as you can imagine, one day I will have to work. unless I want to live the rest of my life as an "i am always broke" student (which I don't), so as much as i love learning,money is also important, and the only way to get it is by working. So eventually I will have to work, there is no doubt about it, the real question is WHEN. (if you don't have to worry about money, then just keep studying, it is much easier and less stressing than working)
now, as you might have realized by now, you've learned a lot of thing during your undergrad, because as mechanical we learned a bit everything. but you havent leaned anything specific yet. putting it simple you have learned a little about,
1. Mechanical Design and Manufacturing
2. Mechanics - controls
what I felt, is that during my undergrad I was exposed to the surface of those three fields, but never had the chance to get deep into them. and when i say deep, i am not taking about reading textbooks or taking more classes related those areas, which of course is good, but not enough. I am talking about research.
Research is an invaluable way to learn, and gain experience, and it is even better when it is assigned to you by a professor adviser. plus, research gives you some insight into actual working, because it is very similar to what engineer do while working in companies.
following the above messy line of thinking, I decided to do a master, and I do not regret. If after I finish it, i decide to work, MS is useful, i will get paid more and will get better jobs. and if I decide to keep studying... well
I am thinking about it now. if I proceed to PhD, then I will work as a researcher after i finish it for sure, and then retired when i get old and work as professor at the university. From what my professors and adviser have told me, it's a nice life, and becomes more relaxing as you get experience.
My adviser professor worked for 20 years as researcher after his Phd, then retired and right now works as a professor, he has around 40 students, 5 laboratories, and does research for some national and private companies.(actually we, his students, do the research, he just tell us what to research and do experiments about, checks our results, and talks to the companies) he is rich as hell, and he is only getting richer. he seems quite happy.
So I what I am saying is, Do a master. it is faster and easier than undergrad, and you will get better jobs after your finish it. and if money is an issue for you right now, just endure for two more years, because after the master you will recover the money that you could have earned working with just an undergrad degree and more!
Doing master is fun, you meet a lot of people, hear experiences from other master or PhD students who have worked before, and have a more relaxing life than workers.
Keep studying mate, education is the key to solve everything.