I ended up never buying either a G5420 or G2420. I was considering it, and so comparing them hands on IRL prior to pressing the buy button. 

In the particular examples in store at the time -it was my local music store in a regional city here in AU so a smaller guitar store, although they had a good range of Gretsch guitars, they didn't have multiple examples of each model nor the exhaustive model range of a large city multi store chain. The G5420T Electromatic I compared with the G2420T Streamliner I saw was superior in attention to finish, some aspects of build and detail quality as well as of course, the pickup models and with different accessories like the plastic knobs fitted to the Streamliner. IDK about the internal Electronics although as you see in a moment, AFAIC that latter is largely academic unless there's an audible issue. 

Both Gretsch I bought previously were double cuts, one top of the Electromatic range, so my expectations were set high. 

Since I last posted in this thread, I've acquired another Gretsch. This time a Streamliner, another double cut but this time a semi-hollow with V-Stoptaill and fitted with Gretsch FideliSonic P-90s. Long story short, I had lusted for this guitar for a while when it came up at a too good to refuse price. So it was an impulse purchase sight unseen other than many online demos and Gretsch's website purchase.

The model is their G2622-P90 Streamliner, which sadly appears to have since been deleted from Gretsch's 2024 model lineup. Although I was happy to compromise on the differences of the Streamliner vs Electromatic builds because of the price and desire for those FideliSonics, I was surprised by the build quality and finish of the example I received. WOW! Accessories and detail things like the different headstock logo notwithstanding & modest yet unique and stylish inlays, finish and overall build quality wise it was as good as both Electromatics. I can't fault this Streamliner's tuner mechs either. I would buy the same guitar again in a heartbeat.

I had planned to swap out the Streamliner's selector switch and knobs for chrome Electromatic ones, but TBH I liked the tasteful retro cream plastic knob and switch tip matchup with the cream pickup surrounds fitted OOTB to the dark Claret Burst stain body from the outset. I had already sourced the chrome Electromatic parts, but in hindsight decided not to fit them saving them for another project as this guy has done here with his G2420 , if I ever get one with black plastic knobs.

Now to the essential part. My Streamliner plays like a dream guitar, feeling every bit as good in my hands as either Electromatic. I find swapping between the Classic "C", thin "U" and soft "C" neck profiles of my three Gretsch instruments a complete non-issue. The frets and edges of the Streamliner are well finished, as is the entire guitar. The FideliSonic pickups are fantastic to my ear for my wants & needs, tonally the primary reason for my purchase of this guitar. 

This demo demonstrates the G2422T-P90 version (w/ Bigsby), but I included it as it offers an alternative superb tonal demonstration of quintessentially the same guitar and pickups.

On the basis of my subsequent experience I wouldn't hesitate for a heartbeat to buy any Streamliner if I liked the tone of its pickups and it was the body/guitar style I wanted. Ultimately the experience is subjective, but I hope that supplies info useful to you.