Before you decide, take the following into consideration:

1 - How near are you to your summon SR pull? If you think you're near the SR pull then just wait til then - you may or may not get King of Hell.

If not, then:

2 - Why do you need KoH? If you need it for a team and you think you can do great in SWB and/or (x)GNW, then go for it. Your power may decrease but just note that power is not everything in this game. You may lose out some of your primary stats as a result of selling your summon runes but it doesn't affect your secondary stats - so you will still have your initiative and/or control advantage (assuming you have). Furthermore, ninjas, strategy, and lag are basically the deciding factor for a match (unless there really is a huge gap in power) so don't worry about losing some power so much. On the other hand, if you only want it for bragging rights, like I did, then don't. It's not worth it.

Also, if you're planning to wait for the new frog summon China has, then my question for you would be, "Is it worth the wait?" Like I said, if you need it for a team and you think your SWB performance will be affected positively, then do you think waiting for a better summon for an indefinite amount of time just so you won't lose out some 5k power would be worth it?

Players can only give you advise but ultimately the decision is up to you.