As someone who did the foundation year plus master's without the year in industry, and am now doing a PhD at the same uni, and also knows someone who did a gap year, is doing the master's, and the year in industry: I think it's mostly down to the individual, though there is also a bit of luck involved.
I chose this route because I felt I would be too old when I finished already, I was still gonna see two groups of people graduate before me, but I was interested, enthusiastic, and successful enough to go for a master's. I was lucky in that most of my home friends took a gap year, a lot of my friends from the start decided to stay in the uni city and in uni areas too, and then a lot of my coursemates did both the master's and year in industry. My friend was also apprehensive about doing a year in industry, and from the sounds of it if he hadn't had such a good time there then he would've regretted it. We also had the advantage that our course had a lot of group work, especially in later years, which meant we were constantly meeting new people and it was easier for yini students to integrate.
It is fairly mentally difficult watching people's lives progress while yours hasn't quite reached that point yet, and it can also be socially difficult watching your friends all move away and struggling to make new ones because you don't have the common situation with everyone around you like in first year. For years in industry this is even more difficult because you are mostly out of the loop for a year. The moment when you realise you're in your mid 20s is also a bit depressing but you'll get that whether you're still in education or not haha
In terms of the advantages of the year in industry, it's quite difficult to say. The usual line I've heard from employers where it makes a real difference is that a bachelor's with YINI is miles better than a bachelor's alone but in the long run a master's has other aspects to it, especially for international companies, where an MEng is going to be considered roughly on par if not having a slight advantage. In terms of MEng only vs. MEng + YINI, the YINI seems to be used as more of a tie-breaker. From a bystander perspective, I've seen people with 2.2 and 2.1 bachelor's who've landed dream jobs and those with 1st class MEng + YINI that struggled. Also if you're tending towards academia then it doesn't really make a difference, if not being a negative as it may prevent you from doing summer research placements at the time when you should. Thinking statistically, it's difficult to compare the different situations, someone doing a master's and year in industry is already likely to be higher achieving and more interested in engineering, obviously that will mess up the numbers.
My advice is that you have a few years to settle in and decide, you don't really need to decide anything until the beginning of 3rd year of your main course. If you want to do it and you're feeling up to it, then go for it. If you're not sure then you can always come back to us for more advice