If I was to go for low noise 2nd RF 87u keyboard, if I have a lot of typing to do then I can't justify anything other than the 87u silent. The variable will help my pinky's and the keys will be lovely, mushy and silent.
If it was purely for thockness, I think the 87u 45g has plenty of thock. If you are in love with the 55g you may find the keys too light, you will require some adjusting, especially the 30g keys on the silent. The 45g uniform may be a good compromise.
Bearing in mind I have headphones on half of the time I would still opt for the 87u silent because I don't think I can go above 45g and to me the thockness isn't as important as comfort. My 660c has a heck of a thock, I love it even more than a RF 87u 45g uniform although it is not as awesome. I have to say IMO the RF87u silent is a delight to use for 8-12hrs. Day in, day out.