I have been looking over this hundreds of times over the last 2 years.
From what i have geathered it seems to end up with around 245hp you will have to spend around £5500 minimum regardless b20 or k20.
-The B20 will be almost maxed out as a road engine and keeping good reliability at 245hp. You could push it up to 255-260hp but that would be a quite high comp big cam engine. With of course prone to be less reliable. More of a track focused setup
-Staying B series means you will still have the same setup engine bay and not have to adapt anything really. If you can change a head gasket and clutch on a B series you can fit a b20. You get to keep and re-use most of your original parts. You wont have to worry about any fitment issues of other B parts as its still the same setup.
-The k20 will be sat happily at 245hp with just bolt on mods if its a Jap K20, Uk ones are of a lower tune and generaly seem to need at least JDM cams to get past 240hp. But to push it up to 260hp you would only need a bump in compression and some mild cams. With that the motor would still be quite useable and reliable. With the option of pushing more or easily supercharging if wanted. Just to point out though that K20's are "More" relaible than B20's but they are not indestructable! Have heard of plenty of them gone bang aswell.
-Going K20 means you have to chop an engine mount off the chassis and fit a new one, adapt shafts to fit, adapt the engine loom to connect to the K20, have to get a manifold that will fit while swaped into an EK chassis, get a custom exhaust to fit, get a k swap intake kit or make one.Im not 100% but im prity sure you have to get a half size rad and make it fit on the N/S. Then you also have the hassle of selling all your old B series parts. (Big pain for me being in the Isle of man I would have to ship everything to buyers at much extra cost).
As for engine caracteristics.
-The K20 will be alot more rev happy and have a peakier power band up the top end. From almost all the Dyno torque plots i have seen of K20s they seem to have a dip between 4500-6000rpm but pull like a train from 6000-9000rpm. Even when tuned. I have only seen a very few with straight Torque curves.
-The b20vtecs have a longer crank throw so are a generaly a torqueyer engine. They usualy end up with a flater torque curve with more from low to mid range but they generaly run out of their power curve at 8000rpm (still quite a high reving motor though).
Its all down to personal preference realy. Im still yet to decide
Don't want to go K20 and want more low mid torque. Go through all the hastle of selling all my B series parts (as i have tones of spares, ie gearboxes LSDs, shafts, heads, intakes, exhausts and much more). And Faffing around adapting my Civic to fit a K20.
Don't want to go B20 and want more revability. or just more power.
Me personaly Im steering towards B20 as its the easiest option for "me". If its not fast enough il just have to invest in more grip and shorter ratios.
But for somebody who has just bought a civic and wants a decent amount of power and reliability, quickly and doesnt mind spending around £5k-£6k then just do a K swap and never look back.
If you only have a bit to spend and dont want to spend mege money then just go 1800, get a spoon ecu and make do. (What i done a year ago, wasn't saticfied
)
P.s a k20 is said to be slightly lighter than a B series setup so wont make any advers effects on handling.