Number will always be better than letter when it's more than one variant especially with buses as unreliable as the 38s pre Covid. In the City Centre especially when multiple buses come at once it's easy to get it wrong whereas if the numbers are different it makes it so much easier. From experience, the 38B and E cause some people a bit of a mix up because of the different routes through Carntyne.
It's the same with the 1s too. Witnessed confusion between the 1/1A with people not realising which goes through Bellsmyre and the Vale of Leven Hospital in particular. Seen it once or twice too with the Golden Jubilee hospital in Clydebank because the 1D doesn't go near it but the 1/1A/1B do.
It's partly a cultural thing - e.g. there have been very few "suffix" services in Edinburgh during my lifetime - quite a few of the daytime ones have been services from West Lothian (e.g. 16A) or Fife (e.g. 55A) - the majority of LRT/ Lothian services that I can think of were evening/Sunday variations (e.g. 4A) - there were some daytime suffixes (e.g. 15A) but the 43 being renumbered 44A is the only daytime one I can think of in recent times (could be wrong, of course, now I live down south, but that's what I can remember)
Whereas Glasgow has had frequent daytime services with suffixes as long as I've paid attention to Glasgow services - back to the days of the Kelvin 5A and Strathclyde 62A competing on the Easterhouse corridor. The only break with this would be the Overground era - I don't think that the initial Overground services had any sufixes other than I think there was a 45A (maybe an early morning service for postal workers?) but there were "A" versions of non-Overground routes like the 42 so it wasn't a blanket ban on them
I think it's going to be one of these things like a "national" livery versus a "local" livery or route branding - some people will have a preference but we're really all guessing what works best of applying our personal preferences.
My take on it (and it's only a hunch/ preference - this isn't science) is that a suffix works well when the vast majority of the route is the same and the vast majority of passengers will be able to board the first bus that turns up. On the southern side of the 38, my hunch is that most passengers on Jamaica Street will board the first southbound service (i.e. going no further than Giffnock), so it makes sense to have them all under the umbrella of being some kind of 38. Maybe it's less useful northbound, since the "split" at Alexandra Palace means that I'd guess that a higher proportion of passengers will want one particular "branch".
Having all services under one "umbrella" feels like it makes more sense when most people will be taking any bus since they are heading for the "core" section. So you can board any "38", it keeps it simple - you only have to remember one number. Whereas, if you have the 35/36/37/38 then passengers for the "core" have to remember four numbers - and then you get confusion over whether the collection of thirty-something numbers includes a 34 or 39 (okay, there's no 39 at the moment and the 34 doesn't share any stops with any of the 38 corridor, but you know what I mean = giving people several numbers to remember introduces confusion over which numbers to go for and which ones to let go - some people may only get a (every ten minutes) 38 rather than trust a half hourly 35, which may mean bunching of passengers.
But, as I say, this is guesswork.
However, I think that the "1" corridor makes sense for the frequent Glasgow - Clydebank - Dumbarton service but I don't know that the 1C has as much in common - maybe I'd have the 1C, 1D and X4 all under a different "umbrella" (e.g. "11" given the shared sections with the 1 - given than the single digit service number already in use) since they have a common route from the city centre up the A814 but don't go as far as the longer distance "1" services.
So, if we were keeping all of the current routes in Glasgow, I'd say that the suffixes make sense on corridors like the 4, 34, 57 and 89 (since the vast majority of the route is the same, the vast majority of passengers will be boarding the first bus) but maybe makes less sense on the southern half of the 7 corridor - the 6/6A along Great Western Road is probably borderline IMHO.
Ultimately though, you're always going to have some members of the public confused by similarly numbered bus services, it's unavoidable, which is why we should try to list changes where possible - and it's probably better to have the occasional (well planned) "big bang" like Overground and Simplicity than to constantly change services.