To me it sorta depends if you want to go the LSW or LMG route. RPKs are the quintessential LSW and by far the most common LSW, but for good reason. V3 gearbox, large amount of external customization options, smaller and lighter than pretty much all LMGs and a lot of LSWs, its just easy to work with.
When it comes to LMGs the A&K 249 (old style or middleweight) is the starter of choice by far. Generally v2 gearboxes do not hold up too well to the increased strain of LMG fire (although in the UK lower power limits will help some) so the LMG gearbox found in most A&K replicas is the goto. Now a 249 build can be a simplistic or as complicated as you want - for instance mines now approaching the $1500 mark but a lot of that is unnecessary. A 249 *can* be run stock, but personally I consider the Bullgear hop unit a must have on any LMG they support. Keep in mind the box mag will die eventually - some people get a good few years out of theirs while some people's die almost out of the box. Bullgear does make inserts to fix this problem and I consider them upgrade #2 after barrel and hop work. They increase mag size, are massively more reliable, have tuneable feed speeds, wind with trigger pull and a button, and some of their standalone mags look ace (like the circular mag I got recently.
For performance I guess they'll both preform similarly OOTB, and Id do the same upgrades to both before I would personally field either (Retro Arms AK hop, new barrel and bucking for RPK, Bullgear hop, new barrel and bucking for 249). Another thing to keep in mind, a 249 (besides the featherweight which I do not recommend) will come with a box mag while a RPK will come with a high cap, meaning theres added cost to either grab more mags or a drum for the RPK. At the same time, you can easily run high caps or even mids in a RPK, and while it is possible to do the same in a 249 they dont usually work perfectly in one. I personally own probably around 15 different LSW, LMG, and GMPGs by now and I dont own a RPK as to me theyre just everywhere and I am a bit fancier haha. Personally if I went LSW Id be grabbing my L86, Galil ARM, QJB-97, or AUG HBAR before I went for a RPK, but again thats just personal preference.