I suggest a power box made specifically for the job, rather than a TENS. TENS 'may be' OK when starting out, but I certainly need rather more 'poke' than any of the six or so TENS units I have tried.
The ElectraStim box works fine and does include one thing that I feel is important - it can be set to change the level, over time, automatically. That way the feeling changes with no intervention by the user. Boxes like the PES can not do this.
Another option to look for is an audio input for line level or microphone. That way you can stim to music or even get 'jolts' from the tap of the heels of a Mistress as she walks around the room.
If you want to get the best get an Erostek ET-312'. The 312 is probably the most expensive unit available but it is certainly the most versatile. It has 18 two-channel routines, 12 normal, 3 audio, and 3 'tri-phase'. You can also get software to let the user program their own stimulation routines, either from scratch (tricky) or using a built-in one as a starting point (easy).
The 312's channels are isolated, like several other power boxes, but there is the unique option for tri-phase output. This gives stimulation between three contact points from the two channels - A to common, B to common, and A to B.