I've never found a 'bad' PU sealant in terms of ease of application. I won't pay Sikaflex prices & shop around for cheapest but I suppose you can't really tell unless you leave it for about 10yrs & see if joint is still good. Used some cheap stuff on my caravan about 4yrs ago & its still good. Check sell by date on cartridge whatever you buy, out of date by much & its often unuable.
Have used puraflex 40 for about 3 years now, really good product.Only trouble I ve had with it was when I used some that was out of date. Had to go back and Overseal caravan again!.As has already been mentioned buy from it from Toolstation for about £4 ish a tube. Met some one the other day who was buying it from a car spares supplier who sells it for £9 a tube!. Dave