For a tent I'd use the Fabsil. It sprays on and in this weather it dries quickly. Try to do it on a calm day though; you really don't want to be breathing it in. There is a 'paint on' version too, but it's more labour intensive, more expensive and the spray can makes it easy.
Each to their own on that though.
Nikwax is more for clothing. Well, I use it for clothing, not my tents, if I make myself clear ?
Fabsil's great on stuff like the hems of trousers too if you're ploutering through long wet stuff, and rucksacks and the like. That wee spray is awfully handy
If the tent is an old heavy duty cotton type one, I'd use the paint on Fabsil though. We have an old Black's Niger up the loft. It's bombproof but it has the same issue that all of that type have, and if you touch the inside of the tent when it's sodden and pouring outside, then it'll leak. You have to run a finger down to the drip edge from where you touched the canvas to stop it dripping through on you. That's the kind of tent that I'd use the paint on fabsil (or I'm pretty sure Screwfix make something similar ) to waterproof.
Cotton swells in the wet and it stops most water getting through, but that's dependent on the weave, (Ventile is brilliant, but way too expensive to make a tent from) the construction and the use of the item. Polycotton is generally not so waterproof.
M