If you send a few SMS each hour, the SMS Server Tools are the best solution for you to build a very stable SMS Gateway.
But if you send thousands of SMS each day you should ask your phone network provider for a SMSC Large Account. It may be much cheaper for you. On http://www.bai.de you can buy software for Large Accounts.
E-Plus does not offer a large account.
Performance note: Sending a message takes about 10 seconds. If you need a faster solution you can use more than one GSM modem or you can use a large account.
You can send messages with one single GSM modem to destination in all phone networks but this may be expensive. If you use one or more modems for each phone network provider you can save a lot of money.
If you prefer a large account think about this: In germany each large account sends messages only into its own network. And E-Plus does not offer large accounts.