I' vote for the East Side...I don't have nikkos but a couple of Endless Summer and others...I am in zone 6b and my hydrangeas would never survive a near full day sun till 4:30 pm..

as for pots or in the ground...hmmm...that;s a harder one...since your plants are in 1-gal pots, these are 1.5 yr to 2 yr max plants, and pretty small in size...Nikkos in time will get quite large - 5 ft tall in 5 yrs?...but since they are in a more managable size now, and with zone 5, the issue of overwintering becomes a more intriguing problem...maybe you can try them in pots THIS YR, allow them to grow in a bigger plant, with a more extensive root system, overwinter the plant in a shelter location, like an unheated garage...then next yr, you can plant them into the ground...being in the pots this yr, you can move them around and test out the spots where the plants are the happiest...btw - the 12 inch pots will be just enough for THIS YR only...

if you ever want to grow hydrangeas in pots, 14 inch is the min...16-18 inches would be my preferences...and I like smaller varieties - nikkos is a bit too big - Endless Summer,Cityline, and some Forver and Ever rebloomers will be ok...the key for success for hyrangeas in pots is this... every yr, in early spring - mid Feb for me -, when the plants are still in full dormancy, and the soil being quite dry, pull out the plants from the pots, and cut off at least two inches of root mass all around, be brutal and forceful but this step is essential to keep the plant happy for the growing season (the plants are dormant, and they would not feel a thing)...then put the plants back in the pots, fill that with new soil mix, and lightly water the plant with 3-4 cups of water, just to make the soil a bit damp...then put the plants back into their shelter spot...take them out when they till you the beauty sleep is over..or after the last frost day in your area...maybe late may for zone 5...this way you are very likely to have a smaller plant, but one you can control the soil acidity (thus the color of the blooms) a lot easier...and overwintering in an unheated garage with these pots are very easy to do...

How would I know my variety? I had a Hydrangea and it looked sickly. So last year I transferred it to a 5 gal pot and it perked up and bloomed nicely! If it's a Nikko, it can't remain there without a trim next winter? I'll try the internet with pics on varieties!