No... neither of these prepaid services utilize SIM cards. The "cards" that are mentioned with the prepaid services are refill cards, not SIM cards.

GSM technology is not used Japan, and most Japanese mobile phones do not utilize SIM cards. I'm aware of only a couple of phone models that were sold in Japan that use SIM cards, and these operate on 3G service in Japan and GSM elsewhere.

The best choice depends on your needs. Keep in mind that prepaid and rental rates are not cheap when making a call. Rates are typically 90-105 yen per minute.

Softbank's SIM rental is 105 yen per day. Calls are 105 yen per minute, and incoming calls are free. This service works well for me as my primary need is to be able to receive calls that originate from Japan or the US, and I have an unlocked phone that supports 3G service on the 2100MHz WCDMA/UMTS band. Since the SIM is assigned a Japanese mobile number, callers in Japan dial the number just like any other mobile number. For calls from the US, I forward my US mobile number to a call forwarding service, which forwards the call to the Japanese mobile number at $0.28/minute, far less than the $1.99/minute than the roaming rate charged by T-Mobile.

As for data, I don't have experience with data services through a mobile phone, but when I looked into it previously I decided it was too expensive. Wifi may be your best bet. The last time I checked, there were a couple of reasonably priced, widely deployed wifi services available.