What's your fishery like?
If you deal with shakers (small undersize salmon) or other small bycatch, then running braid with a mono topshop is ideal. The braid will telegraph more action to your rod tip, and make it easier to identify small fish on the line. Braid is also going to give you better sensitivity, for whatever you think that's worth.
I spool my salmon moochers with backing, to braid, to mono top shot. I like this best overall.
If you're derby fishing and you tend to have larger fish in your fishery, pure mono is likely to give you the best advantage, as the stretch of mono will make it easier to land fish, especially if you're not experienced. Even if you are experienced, imho it gives you the best shot at landing a fish. After you get used to braid, pure mono is kinda like a condom. Less sensitivity. Maybe you like that, maybe you don't. To each their own.
If you find yourself trying to keep your fish away from a seal, braid is absolutely better. Not that you'd be assured to get it away from the seal, but your chance does go up. It's better for snags too, but if your mooching on the troll bottom snags should never really happen. As others have mentioned, you don't need to replace the braid often, especially if you run 50-65lb braid.
Keep in mind, that braid under 50lb is might be difficult to clip onto your downrigger (for whenever you want to clip on past your top shot length). Topshot length varies by angler, but 20' is minimum, up to maybe... 60'? It varies, adjust to your preference for your fishery, depth preference, and braid lb test rating.
As an aside, keep some salt water lathering soap on board. If you use fish attractant and it gets onto the pads of the clip, only soap is going to save you. AP Tackleworks makes some (wish I could find it in stock!) and I'm sure others as well.
I'd recommend backing, 50 - 65lb braid, and a 30' topshop of high vis 20-30lb mono for the top shot. Save your flouro for between the flasher and bait/lure. I'm not convinced flouro makes a difference in our waters, as it's not exactly crystal clear, especially at depth. Also, replace your flouro at the slightest nick - it's not as forgiving as mono.
Also, you want an FG Knot for your topshot, it will go through the rod guides easier than double uni. Double uni is much easier to tie though, so tie whatever gives you the most confidence.
Hope that helps - tight lines!