Should I use my 21.5" Dell 1080p external monitor OR buy a refurbished 2018 MacBook Pro 15"

[Hieveryone]

My current setup is a late 2013 2.6/8/256 13" rMBP connected to my 21.5" Dell 1080p external monitor.

The resolution isn't as good as retina of course, but it is a bigger screen than the 15" on the MacBook Pro (and the cheaper option!)

I'm quite conflicted, and would love some advice bc the 2018 refurbs must be coming soon and I don't know what to do.

 

[DeltaMac]

(?) --- I don't really understand your conflict.
You already USE the Dell display on your 13-inch MBPro, right?
The 21-inch display will still be larger on a 15-inch, so do both: Keep the external display, and continue to use it with your new 15-inch refurb.
You may need a different dongle to use that monitor with the new refurb, but that's the easy part.