Travel with Suitcase or Backpack?

I have traveled in other countries before with a backpack ... but I think Japan will be different. It is not quite the "backpacker" country, like in Guatemala or Australia.
Please advise:
Medium-size SUITCASE or Kelty 4,500 cubic-inches

My travel plans:
Days 1-7:
Arrive Osaka airport, travel to Shiga-ken, travel to Kyushu for 3 days (Fukuoka, Beppu, Oita), back to Kobe, Kyoto, then to Wakayama (Koya-san),
Days 7-14: Tokyo.

I figure the pack would be more convenient to actually carry in train stations, but on the other hand, I will have to have different clothes for different occassions, including a dress for a wedding reception, a blazer and slacks for a professional meeting ...

I've heard of coin lockers, but in this case, it is pointless because I will HAVE to have my belongings with me all the time.
Any advice will help! Thank you!

I traveled with my rucksack, 30-60L (3400-5200 cubic inches) sorry can't remember actual size, witch looks little shorter then the �eKelty 4500' I saw on the web but width/depth about same.

I was able to fit the bag into the overhead shelve, on shinkansen and local trains, with minimal force. As for traveling around the cities, I traveled Kyoto/Sapporo/Hiroshima/Mt Fuji/etc (for 14 days), I didn�ft feel any undue glances that would make me feel uncomfortable.
If you bring a smaller bag, backpack, then you can leave your large bag (with clothes) at station/hotel and sightsee with your camera/passport/weather related product (hat/sunglasses/water/umbrella/etc).

A backpack is okay to travel with in Japan and not many people will care much. If you choose to travel with a backpack, when on a train/subway/bus or on the train/subway platform, do not wear it on your back. Place it at or near your feet. Even if the train is some-what empty(no seats but a lot of standing space), still place the backpack at your feet. When a person is wearing a backpack on his/her back, even the smallest turn, will cause the backpack to swing around and this has been known to hit a person standing near. Wearing a backpack on a full train/subway is a "no-no", so are large bags on the overhead racks. So if you have to travel on a full train/subway, try to ride at the back end of the last car, there should be more space there, for your backpack at your feet.

I think a backpack is easier especially for the train.