As much as I like Karel, Harken is the superior choice regardless of mode. He comes with a far superior weapon (the Brave sword), better class (the use of axes trumps an additional +15% critical hit rate), better support options, and much more solid stats. On Hector Hard Mode, there's simply no question that Harken wins due to his huge stat buffs, but even when playing other modes, Harken outclasses Karel in virtually every way.
It really depends on the mode and who you've been training. If you have been using Guy than get Harken. If you've been using Raven get Karel. If both get Harken for the Brave Sword. If you are on Normal Mode it's really up to you. But if you are on Hector Hard Mode, get Harken because he gets stat bonuses.
Harken gets stat bonuses in HHM (Hector Hard Mode) so if you are doing that then definately take Harken. Otherwise I would still recomend Harken because his supports are slightly better in my opinion, he comes with a brave sword, and he is easier to get.
Karel and Harken, as well as their weapons are equal among the difficulties except for Hector Hard Mode, where Harken immediately gains the stat benefits that make him significantly stronger than Karel.
So, it really depends on your preferences, but if you're doing HHM, then it is highly recommended you get Harken.
Well it depends on what you want. Do you want an extremely fast character with a higher chance of getting a critical hit(who is also harder to get), or do you want someone who has nice decent stats who comes with a really good sword(who is easier to get). I prefer Harken over Karel personally.
For me, it mostly depends on which weapon you want to get, as I don't like using prepromotes(without Hard Mode bonuses).
If you're playing Eliwood mode, depends on whether you're using Raven or Guy. If you're using Raven, go with Karel. If you're using Guy, go with Harken. If both/neither, go for Harken because the Brave Sword is generally better than the Wo Dao. However, if you're playing Hector mode, no doubt Harken's your man, as you'll get another Wo Dao when you recruit Karla. Not to mention Harken's stat's overwhelm Karel's in Hector Hard Mode.
Personally, I mostly end up using only Raven, so I go for Karel. In Hector mode, however, I sometimes just forget about Swordmasters and go for another Hero(Harken).
Harken is way better and you get a Brave Sword out of it. The only problem is that Raven and Karel are just so much better than Guy. So if you want one Swordmaster and one Hero get Karel, and Harken if you dont care if you have two heroes.