I have built the Tamiya Mk V and the new tool Airfix Mk I (which I assume is substantially similar to the Mk V in terms of general construction/detail, etc).
Tamiya:
You really can't go wrong with this one. Detail is good, and fit is perfect. A very easy to construct no-putty model that looks good. The decals are not great, so you may want to replace them. Comes with both Vokes and Aboukir filters.
Again, it's hard to overstate how well this kit goes together. Tamiya's engineering sometimes amazes me.
Airfix:
First, make sure it is a new tool kit and not some 40 year old mold in a new box. That said, I found the new Airfix kit to be a little more detailed than the Tamiya kit, especially in the cockpit. Really almost a tie here, slight edge to Airfix.
Surface detail is very good, panel lines are deeper and more pronounced than Tamiya. Again, more or less a draw.
Construction is more fiddly than Tamiya. It goes together fairly well, but really nothing like the Tamiya kit. Some putty and sanding will be required, nothing too major.
Parts count is higher, lots of small bits compared to Tamiya. Decals are MUCH better than Tamiya.
Conclusion: they are both good, but I'd personally go with the Tamiya kit and some AM decals. The ease of construction is just so nice.