My setup is very similar to Skadoosh, as well - but I have the standard 590A1 without speedfeed stock. I do have the Allen shell holder sleeve on the stock though. I keep 5rds of the Winchester PDX1 (1oz slug w/ 3 00buck pellets) loaded in that, while the mag tube and chamber are loaded with Federal 00buck. I really like the way that balances. I'm not a fan of the side saddle because I don't like the added weight up there, nor essentially doubling the width of the receiver. Just seems too fat.
I do have one of those bandoliers hanging in the closed loaded up 25 00buck, 15 3" 00buck, and 10 slugs.
Glock 19 G4 Gray | Glock 17 G4 FDE | S&W M&P 22 | S&W M&P 45 | S&W M&P 9 | S&W M&P Shield 9
Sig P226 MK25 | Sig P229 Extreme | Sig P320 F | Sig P320 C | HK USP 9 | HK USP 45 | HK USP 45 (Tan) | HK VP9
Daniel Defense DDM4 V4 | Arsenal SGL 21 | ASA SBR | Savage Mark II FV-SR
Mossberg 590A1 20" SP | Mossberg 500 Cruiser