I have not used the flexible cutter you listed in the URL... I have
reamed a number of valve guides over the years... My experience has
been that you will find a combination of carbon / lead salt deposits
and slight warpage (sometimes not so 'slight')... So, the straight
reamer at the nominal 0.4985" size will do the best job... It will
remove deposits and ream out the tight spots inside the bore...
Your mechanic knows best, but my suggestion is to do it by hand with a
Tee handle - and lots of muscle... The thought of using a drill motor
may be attractive, but in less than experienced hands it can wreck the
guide and then you will have to have the jug pulled and repaired...

There is still nothing stopping you from first putting solvent down the
guide bores, fresh oil, AVBLEND + MMO, and flying it... Less work and
cost than reaming the guides and doesn't do anything that can't be
undone...
Local FBO and I did this to his rental Skyhawk recently when it
suddenly stuck an exhaust valve and almost cost him a a 100 hour block
rental from customer who needed to scout a number of factory sites
across the southland over a 4 week time period, leaving the very next
morning... This was big bucks down the drain if the airplane couldn't
go the next morning...
A large spray can of WD-40 (only thing we had on hand that night) and a
quart of MMO in the oil saved him a valve job... The ol hawk came back
still humming along... Old chinese adage, "Softly, softly, catchee
monkey"..

denny