For 3 months stay, portable WIFI or SIM card?

I'm going to study Japanese for 3 months in Osaka, and I think I will need connections to the internet. Call is not necessary so I'm confused whether to rental SIM card from CDJapan or portable mobile WIFI

Rental SIM Card :
http://rental.cdjapan.co.jp/index_en_jpy_10.html

WIFI :
http://wifi.tocoo.jp/en/fee-n-product.php
http://www.pupuru.com/en/service/emobile/
http://japan-wireless.com/products.html

Do you have any suggestions or reviews on those providers? I want the cheaper but reliable one.

And can I use my Sony Xperia T2 in Japan?
I've read some questions talking about locked and unlocked phone but I don't really understand what it means. Can you please explain it to me?

Thank you

Okay, I'll bite...

For three months, the cheapest option is almost certainly a "non-tourist" b-mobile SIM card, but the drawbacks are that everything is done in Japanese (so you would need to either be somewhat proficient in Japanese yourself, or have a Japanese speaker assist you) and also payment is by card only (Visa, Mastercard, etc.) so you need to have one and also make sure it is accepted in Japan.

http://www.bmobile.ne.jp/bmsim/index.html

Note that if you are a temporary visitor (i.e., without a resident card) you can only get a data-only plan, not a data + voice one, but I suppose that's fine with you.

A "locked" phone means that it can be used only with the carrier who sold it. This generally happens when you buy a phone together with a contract: the phone is heavily discounted, but on the other hand you generally have a commitment to keep the contract for a certain time, and the phone is locked. If your phone is locked, it won't work with any provider other than the one who sold it, so in particular it will not work with any of the Japanese SIM card providers. If you bought your phone "naked" (i.e., it didn't come with a SIM card and contract when you bought it), then it's almost certainly unlocked.

Unlocking a locked phone is typically possible, but it may be illegal in your country. To know whether or not yor phone is locked and, if locked, can be legally unlocked, ask your carrier.

If your phone is locked and cannot be unlocked, you will have to get a pocket WiFi.