2 .. for the typical Laptop made today you need to check the technical specifications for that laptop to find out the speeds etc. The same goes for Tablets. For Tablets Apple make a point of being current or leading edge --- for Windows the high end units will be current or leading edge; the more economical models are lagging -- which is why it is very important to check the technical specification for the product being considered.
1 .. 3 streams typically means that there are 3 radios and 3 antennas working concurrently to send/receive data. From a technical perspective the radios and antennas are controlled by software built into a chip that can determine how-to work based on information being received from the client NIC. If the Client NIC only works on 2 streams then that spacial software dumbs down the 3 streams to 2 streams.
The E3000 would be the one I would suggest that you consider in view of the fact that no-one has yet released any client that I am aware off that can support the higher speeds of the E4200.--
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