[Motel75]

I think your concerns have been addressed, but it depends what you're looking for. If you're going to make a lot of calls within Germany, you should consider one of the discount chains' SIMs, namely Aldi Talk, Penny, etc., which have per-minute prices of about 15 cents.

However, if you just want to be reachable, don't plan on calling much within Germany, and want a card that you can keep for return visits, you might prefer one from the networks themselves, specifically Vodafone, T-Mobile, or O2, all of which last 15 months or so, are easy to top up in the UK to keep them going, and for which you can create an internet account to monitor. (E-Plus is the fourth network, but topping up can be more difficult in the UK, as it is not present there and does not belong to an alliance.)

Topping up the discounters' accounts from outside Germany will be difficult, although Aldi Talk can be extended to two years' validity by topping up with 15 euros after buying it. AFAIK, none of the discounters offer Internet accounts, and topping up with a credit card is not possible with any German mobile (except for Simyo, and then it has to be a German card).

Coverage is a non-issue for visitors, as all networks are pretty similar.

Note that all international calls from all SIMs (with the sole exception mentioned above) cost an outrageous 2 EUR, a rate that hasn't changed since prepaid was introduced nine years ago. Data rates are also among the highest in the world. Your UK mobile will probably offer cheaper calls back to the UK than your German SIM will, and probably better data rates as well (Virgin Mobile does, for example).