To be fair to Adrian of Speedline, I've seen or rather read some pretty opinionated stuff about his kits so I dont have a problem with him correcting people even if some think it was an advert.
To me we are talking chalk and cheese here. Slipway do excellent kits, I have a Drumbeat of Devon and am putting the finishing touches to an Envoy civvie tug. The instructions for Drumbeat were very good. The instructions for the Envoy less so but, Slipway indicate the Envoy is for more experienced builders. I had a couple of minor issues with the build of both but, I would recommend Slipway to anyone. Just buy the build from their range that's within your competencies.
I've also studied Speedlines site and their lifeboat kits. I think Adrian makes it clear none of these are for a complete novice. But, as with any build, if you are careful and take your time, even a novice can turn out a great build.
As regards price, well some people shop in aldi and some in waitrose. Yer pays yer money and all that. The speedline Tamar is 50% bigger (if my maths is right

) than the Slipway version. But the Speedline version also comes with actual stanchions and other bits and pieces ready made rather than white metal or pieces you have to make yourself. All these things add up.
I'd love a Speedline Tamar and Im certainmost of its within my build capabilities. Personally I'd pay the premium for the speedline not because the slipway version is in any way a lesser kit, it's not. But the speedline is bigger and I like big scale models!
So, what it comes down to is
Can you justify spending the money on the preferred kit
Are your build skills sufficient to justify a big kit (where mistakes are harder to cover up)
Can you physically handle a bigger kit
Scale boats look better at larger scales so is scale appearance of the boat and on the water important.
IMVHO, only the purchaser or in this case the OP really answer the question of which manufacturer to go with.