That can only be as a result of the operator being misinformed, or low stock levels on the E5186; as on a purely technical level, it makes little sense to supply an LTE router on an LTE-A contract, especially if the client is in an LTE-A area!
If you are very insistent, you will probably be able to get the E5186 instead as the option does exist - see below:
It depends on what is going to be more important to you over the 2 years after you sign up : more speed, or more data ?
Provided you are getting at least 50Mbps, then I would guess that most people's answer will be "more data".
Given your stated usage profile, and once you are over about the 50Mbps level, then I'm not sure that more speed will add much more to your internet experience. With a decent antenna setup, and provided there are no other constraints, both the B593 and the E5186 are easily capable of that speed.
On paper, LTE-A (and hence the E5186) do give some "future-proofing" as regards the move to higher speeds. However that move seems to be happening quite slowly. Apart from the problem of limited LTE-A coverage, I've not yet seen anyone on the forum post a SpeedTest result above about 90Mbps with either LTE or LTE-A, and I think I'm correct in saying that all of the peak speeds reported on the forum to date have been achieved with the B593 on "plain" LTE.
Personally, I think you'd get more benefit from one of the promotional uncapped LTE deals with a B593 (either device, for that matter). That's provided it operates as Telkom promises and your station isn't overrun with bandwidth-hogs who game 24/7!
Also be aware, that the "uncapped" LTE comes with some fine print, eg max 50GB of data "abusive" protocols. But from your current usage profile, it doesn't sound to me like you'd have much trouble staying on the right side of Telkom's uncapped Fair Usage Policy.