@Litil It's true that NUSspli won't find all games (basically disc only releases like Call of Duty: Black Ops II are missing) but on the other side you find zero games on WUP Installer. So there will never be a situation where you find a game on WUP Installer but not on NUSspli. Also NUSspli contains a WUP Installer, so you really don't need WUP Installer anymore.
In fact NUSspli is better at installing. One example are disc dumps: With WUP Installer you start WUDD, dump the disc, move the files to the install folder and finally use WUP Installer. With NUSspli you start WUDD, dump the disc and then use NUSspli. So no more need to move the files to the install folder.
A channel is like a Wii U game: You need to install it (with either NUSspli (HBL version) or WUP Installer), then you have it on the Wii U home menu. So you will be able to start it without going into HBL first. Now before the question arises what's better (HBL or channel version) - that depends on personal preferences.
//EDIT: To the first point: I guess you're reading about USB Helper and stuff but to be fair: USB Helper & Co froze in time around 5 years ago. This means you're missing all releases of the last ~5 years there. NUSspli on the other side has all content on the NUS (NUS = Nintendo Update Server. In fact NUSspli has been used to download content before said content had been officially released).