I don't know of any database system that has an artificial limit on the number of operations per second, and if I found one that did I would be livid. Your only limiting factor should be the practical restrictions imposed by your OS and hardware, particularly disk throughput.
The rest of your question (which database is "better") is subject to your implementation and requirements. If you're just dumping data into a bucket a NoSQL solution like MongoDB may be appropriate, and their performance can be quite impressive. If your data is highly relational SQL-based RDBMS systems are the better choice.
With any SQL-based RDBMS you should expect to spend some time tuning the system for optimum performance -- Your database vendor will probably have a small mountain of documentation on the subject, and the difference between an optimally-tuned system and one that was just thrown on the hardware can be dramatic.