Top up vouchers for Fonic are not as easy to get as for other German providers (see
Germany - Fonic - Prepaid Wireless Internet Access for an overview of retailers). Against that background I would regard recharging by credit card as much more comfortable.
If you compare Fonic's 60/60-billing to solomo's 60/1 billing, Fonic is significantly more expensive. An average mobile call in Western Europe takes 1.5-1.9 minutes. So based on this fact Fonic is 5-33% more expensive than solomo. I think this discrepancy should justify solomo's price premium of € 5 for the SIM card.
Regarding network congestion, the eplus network, which solomo uses, is really problematic, but usually that's only reflected in low data rates, while SMS and phone calls are not an issue.