Sanji didn't choose it, as he said himself "I cannot change what already happened".
So he needs to work with it anyway. He asked Zoro to kill him because of what he may become as a person, this stills being the most important thing for him (as expected). This changes a LOT of things for Sanji's character
Now it doesn't changes his usual look and fighting style like the RS would do, and both are amazing
It opens room for Oda do lots of things in the future strengthwise, like Sanji already did, by using the Exoskeleton he was able to boost his flames stronger, something that a regular human body couldn't, even his body already being tough to resist Diable Jambe.
The Exoskeleton awakening opened more rooms for Sanji to grow quicker, and do things that no one else can do. And it still made clear that:
- Sanji can boost his flames at will, having a regular human body is the only limitation
- His CoA training matters a lot here
So as you can see it is a combination of many factors, but Sanji, his training, his flames and his skills as a fighter still being the one making the biggest difference for him to grow.
The more Sanji gets used to that, the more he learns about it, he may even boost it by himself (like he does with his flames) and find more about how to use his overall powers to its full extent, in a level much more higher that his family would never be able to reach
The RS not only drastically changes Sanji's fighting style and look, but it is a device that completely resembles Germa and gives him invisibility, by having invisibility Oda wouldn't need to boost other attributes like speed, looking way cheaper to get stronger. Sanji is a martial artist, we need to see him making use of his body to fight and surpass his opponents, speed fits Sanji, CoO fits Sanji, strong kicks fits Sanji, flames fit Sanji. The invisibility has a gag purpose basically