I'm unclear if OP is talking about sitting 11+ for independent or state schools (or both).
What would be best is very dependent on the specific schools you wish to sit for.
If you're talking about state Grammars, then I'd advise having a good tutor - the tutor will specifically prep for the specific exams to be sat. Bear in mind the good ones get booked up very early.
If you want to go to an independent that has a linked Prep/Junior that guarantees entry to most of the Prep/Junior pupils, then going to the Prep for Yr5/6 could be a good move if you worry your child might be borderline on the exam.....but do check their specific requirements - some might only waive the entrance exam for those who've been in the a prep for longer than those joining at Yr5, because they tend to make the offers to their Junior students in Yr5 not 6 and need enough time to make a realistic assessment of your child.
And if you're interested in an independent senior which doesn't have a Prep, just be aware that other Preps are often prepping for lots of senior schools and some of that Prep can be rather generic rather than targetted at one specific school and it's specific exams.....you'll often find those parents are often paying for tutoring too because of this.
So actually, on balance, unless you are looking at an independent Senior which has a feeder Prep, I'd say a good tutor (and his needs to be one with experience of prepping for that particular school - not a Kumon centre or similar or tutor who does groups of students all going for different schools at the same time) will give you the targetted help that is needed for the exams probably better than a Prep school. Most of the Prep day will be spent on usual stuff such as humanities, music, art, sport etc etc - all good and part of a broad Prep education but not vital for getting through the 11+. For most 11+ Emglish and Maths needs to be good and often VR and NVR needs covering too. Of course Prep schools teach English and Maths but I'd query if their teaching of just that warrants the fees in comparison to paying a tutor.. Some will schedule an hour a week for VR or NVR in the timetable or in an after school session, but again, is it worth £15k fees and will it be better than a tutor who targets a specific style of VR and NVR as required?
I'd say Prep schools are great if you want a Prep education. But if you're not really after a Prep education but the passing of 11+ There are other cheaper and possibly more effective ways of achieving it, especially if you are also willing and able to put some time in with your DC too.
If looking at Preps ask v specific advice - exactly what preparation do they provide for 11+ in terms of specific skills, for GL or CEM exams, how much time devoted to it, how many different schools they are prepping for and how they ensure everyone is prepped for the different exams they will sit. Yes, Preps do advise parents about suitable schools to apply for etc and that can be useful and is part of an overall Prep education package, but is it really worth £15k if you can't say money is no object. Most sensible parents have the ability to go to some open days, and speak to a tutor or ask for an assessment fromma tutor to get a sense of if applying to a certain school is realistic.
I think Preps have a place for families with plenty of cash who want a Prep education, but the idea that they do something really magical in getting children into competitive day schools, that can't be achieved by interested, educated parents and possibly a tutor isn't quite true and a bit of a myth that the Preps themselves and their parent bodies perpetuate. Yes, a wholly unprepared state school child will be disadvantaged even if bright, but being at a state school doesn't mean you can't be well prepared and if bright, have a good chance of a place. The top independents all take a decent number of state children. The vast majority have been prepped by parents or tutors.