Choosing a Hector

In my opinion, since you use him fairly often, it would be worth it to merge into the new IV. You can use a threaten skill or something else in the C slot if you don't like the suboptimal threshold for Panic Ploy, but I think that close to 50 HP is still a fairly good threshold, especially since so many units like a -hp bane. +def is one of the best boons and you will be patching up his attack at the same time. Therefore, I think you should merge unless you desperately need Distant Counter on, say, Myrrh or Effie.

If you are planning to merge, do the +def. That is much better than +hp-atk. Even with 49 HP, he is still high enough to affect the most crucial groups (horse/flier emblem) for ploying.

I'm gonna say it depends on how much you use him. From your post I'm guessing its a lot. You could always merge the old one into the new one and then just grind up the SP for all the old skills and end up with a better version......But man that Distant Counter fodder potential is tempting.

Obviously your old Hector isn't going to fall short anywhere in the game except maybe super high end T20 arena and even then I'd still say it probably isn't much. Not to mention, again, that Distant Counter man. That's one hell of an attractive skill for SI. You could just go through the pain of investing into the 2nd Hector and fodder off the old one to get another unit with DC, which if you're willing to do that, I might just because DC is that good of a skill.