reflexes is a mixture of recognizing a danger then being quick enough to avoid it. but first, the sharingan doesn't really slow down people it's more just the user is able to see things before they happen thus it makes the the opponent seem slower. the fact is both of their reflexes work in similar ways. naruto is able to sense things all around him, and because of the added agility that sage mode brings, he is able to react to them. remember when he fought beast kage. he activated sage mode for the precise reason it would give him better reflexes to dodge at the last second. he was going head on with him, thus no reason to need to sense where the attack was coming from. he just needed tp be able to dodge at a point where beast kage couldn't react. thus his reflexes are great to react to speeds that the raikage was going. add the required multiplier to his current power and you get a glimpse into why people praise his reflexes.
sasuke, because of the sharingan, is able to see things before they happen. the thing is, while his sharingan is able to make him see whats gonna happen, it doesn't make his body faster. case in point the VOTE fight. naruto powered up again and sasuke couldn't dodge his attacks. so he has to rely on his own natural reflexes, which will only be so great on their own without some way of multiplying it. also, the rennigan doesn't add on to your precognition prowess. it will however add to your line of sight if using the ability to control other people with chakra rods.