I have been using Mac for a while now and prefer it to windows based machines, just for simplicity. However they are more costly and this may not be worth it for you, in addition to this it is challenging to upgrade the specs on Macs at a later date.
I would certainly look at upgrading the RAM on your current machine, with photoshop and Lightroom these days a minimum of 16GB is needed and ideally 32GB or more. Whichever path you choose RAM is a necessity with Lightroom and I personally wouldn't look at anything less than 32GB.