Nano SIM card cutters are available for $6.28 from US based sellers including domestic shipping and are an essential tool for travelling iPhone owners:
http://www.ebay.com/sch/i.html?_from...rt=nc&LH_BIN=1
In order to make an informed decision you might want to review all four German operators' coverage maps:
Telekom (including Lebara sans LTE)
Vodafone (including Lycamobile sans LTE)
eplus GSM/UMTS
eplus LTE (including AldiTalk, blau, simyo)
o2 (including Lidl, Fonic, sans LTE)
Keep in mind that among German operators only eplus allows prepaid subscribers onto their LTE network regardless of their tariff plan while Vodafone offers explicitly
LTE-enabled bundles starting from € 10 for 500MB/month.
Given that eplus has started its LTE rollout only last year using a comparably high frequency band (1800MHz/LTE band 3) their LTE coverage is still very limited and concentrated on the large agglomerations. In contrast Vodafone has pretty far-reaching LTE coverage especially in rural regions thanks to the low frequency band used (800MHz/LTE band 20).
Following the acquisition of eplus by O2 both networks will undergo an integration process which will involve decommisioning of many redundant cell sites and probably opening both networks for biliteral national roaming. Actually o2 has recently started a closed beta test in a rural area where o2 selected customers can now roam on the eplus network. In the end of this integration coverage should significantly improve for subscribers of both former networks.
The iPhone 6 (+) supports all German GSM, UMTS and LTE frequencies, regardless of its version (A1522, A1524, A1549, A1586), so there's no hardware limitation for you.