It seems to come up a lot this discussion.
I looked at both the NEBOSH and NVQ routes when considering the next step. I did the health & wellbeing certificate after not doing any sort of stufy since the early 90's and although the exam wasn't horrible, it's not something you'd put yourself through for fun.
Whatever route you take, as has been said, you need to have the inclination to that way of working. If you haven't had any experience or exposure to H&S gnerally as part of your role, you may struggle getting evidence, but equally, some people who did the NVQ had nothing in place before doing it, and then did the course and actually implemented processes and policies because of the NVQ, so they kind of gained some expereince as well.
The NEBOSH is acedemic and it proves a broad range of knowledge which you may find useful if you are transferring skills. The NVQ in my eyes is also transferrable although the evidence you use is specific to the job you are doing at the time, but the principles are the same as they are principles of H&S you would apply anywhere.
So it really comes down to what your preferred method of learning is versus your level of experience.
Either course leads you to being able to be Grad IOSH status, the only difference to the chartered route being that if you did the NEBOSH you have to produce a portfolio (so evidence like the NVQ), and if you have the NVQ, you need to complete the open book assessment (like NEBOSH). You can gain chartered status both ways.
So if your ultimate aim is to become chartered, then the diploma route will help you get there. If you got chance to also do a masters, then that is a massive bonus. I would suggest your colleague is just jealous of this prospect. Why is having letters after your name anything to be ashamed of? It proves you have backed up any experiecne with knowledge and qualifications and lets face it, how many jobs have your seen in H&S that don't ask for qualifications either to degree level / diploma level or chartered level?
I'd so go for whatever you think will work for you, and for your own benefit, as that is the most important thing int he long run.
Practice makes perfect, apparently.....