Both of them are right and wrong. LTG said that not doing an MSc is 'standard' in STEM because most do an integrated Master's course (and then graduate with an MSci - equivalent of MSc + BSc) and then Klix started talking about the 1+3 route, which is entirely different. She also said that MPhil's aren't research degrees, which they are (as they are entirely thesis based - there is no taught content). However, she was right when she said an MPhil is a higher qualification that a standard Master's. It is still within the Master's range but is considered a senior Master's as it is the most advanced degree prior to a PhD. In this way, although it is still a Master's degree, an employer would consider someone with an MPhil as senior to somebody with an MSc.
You can learn more here:
https://en.wikipedia.org/wiki/Master_of_PhilosophyNow, let's get back on topic:
Kevin, if you wish to do a PhD and you enjoyed your final year research project, then go for it! You must realise, however, that this is a 3-4 year commitment and leaving early will essentially cause you to leave with nothing (you'll receive an MPhil, but this is nothing on a PhD). If you enjoy the subject itself and want to work in that area, you're pretty much guaranteed to have to do a PhD at some point and the lack of a Master's won't really hinder you in any way, so I'd say take this opportunity while you can, as PhDs in subject areas that you really enjoy don't come around too often.
Good luck!