I used to be on Newnet 3GB and it was fine for 18 months. The difference between me and you is I do a lot of browsing and posting on these forums but no mp3 downloads.
I began to hit my 3GB limit, so had to buy a couple of topups. In those days the topup system wasn't quite so good as they didn't carry over if unused, but now they do. Also the short unmetered night period wasn't there.
O2 LLU was on my exchange and by using a rubbish (for away from home) mobile phone I was able to get a free O2 SIM, top up by £10 once every three months giving me 500 free minutes in month 1 and the topup to use in m2 and m3 - so nearly zeroising my BT calls bill - and get LLU unlimited for £7.50 (now vat-adjusted £7.34) per month. The 12-month contract scared me but I had a 50-day getout clause. So I jumped. Unfortunately this option isn't open to you

.
I suggest you try either Newnet Home Lite or Newnet Home M for a few months and see how it goes. Take the monthly contract option and you can always change again to Plusnet, whereas you cannot move from Plusnet to Newnet for
18 months!
If you do go to Newnet it depends how much you want to bother checking how your usage is going. Home M should far exceed your current requirement, but Lite may be all you need. There is a fairly complex (at first) system for buying topups that means if you are happy checking your usage now and then you may very well be fine with that plus occasional pre-buys and still be cheaper than Home M. And the excess charge for not pre-buying is not a lot more anyway, and through the control panel you can control how much over-allowance usage you are willing to pay for in a month as well. There is an automatic emailing system to warn you at 75% and a few times after as well, though it doesn't seem to be totally reliable.
Having said all that - as a convinced Newnet fan/believer which you need to take into consideration - Newnet is in my opinion the better of the two, but Plusnet is definitely the best alternative. You should only be looking at these two, from what you have said so far.