NanoSim is a FastSPICE circuit simulator, while Eldo is a SPICE circuit simulator. Similar tools would be HSPICE and Eldo, or NanoSim and Mach TA (or ADiT).
Both companies have co-simulation between FastSPICE and SPICE, plus digital simulators. The full-blown system for Mentor is called ADVance MS allowing for co-simulation with ModelSim (digital), and with Synopsys they co-simulate with VCS (digital).
Both companies have real customers so it comes down to your preferred EDA supplier and your own benchmark results.
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