In reply to wilkesley:
This feels a bit like you want one of the early Naughties internet-fights-to-the-death type threads
https://xkcd.com/386/ but I haven't used the travel tap, so I can't review it in comparison.
So again, the Travel Tap looks great - it might well be a better solution for a solo backpacker than the Guardian being a lot lighter besides anything else. Indeed if I have a go at the Pennine Way this summer, I might well invest in one.
But as you asked, firstly the tube on the Guardian means you can put it into tiny or very shallow water sources that you can't fill a bottle from. It also has the prefilter on the tube and the backwash meaning it could deal with muddy water that Travel Tap says you need to avoid (or prefilter in some way). Having watched a couple of YouTube videos of people reviewing the TT, I also thought to be super careful you would probably want some other bottle to scoop water up with and decant into the travel tap bottle. The vids of people sticking the bottle into a lake, putting the filter cap on and immediately drinking from it strikes me as a good chance of cross contamination, with water on the outside of the bottle running down to the cap which you drink from. I would have thought at least drying it off with a towel would be a good idea? The reviews on Backpackinglight.co.uk are mainly good but someone mentioned theirs failed rather quickly (blocked up maybe?) and needed replacing. Other people note the problem with the bottles leaking around the cap - again not what you want if the water is really dodgy. MSR claim the Guardian is freeze resistant, which I believe is a big problem on activated carbon filters like the TT. One of the YouTube reviews also mentions it being very slow (he was filling another water bottle from the TT), the Guardian purifies water rather quickly.
So some advantages to the Guardian, some disadvantages - cost not being the least of them. I hope maybe in a year or so MSR will find a way of using the same filter technology but in a unit half the weight and size of the Guardian - a lower price would be good too! That could then be a great filter for solo backpackers, even if the flow rate is half that of the current Guardian.