I'm accessing work email on the mobile phone and will need to turn on laptop occasionally to process some work.

I'm accessing work email on the mobile phone and will need to turn on laptop occasionally to process some work. If I use the mobile phone as portable wifi or tether as mobile broadband, will the speed be as good or poorer than a dongle? just weary of having to plug in dongle each time I need to use the laptop. any input appreciated.

So long as both are the same type (eg 3G) it should be similar. 4G versus 3G should make a difference.

That said there are better chipsets than others and some work/better worse with various telcos equipment, or you might have a dongle with an external aerial. 

So should be the same but YMMV.

In my experience a tethered smartphone (iPhone 4, on 3G network) is slower than a USB dongle, especially when you are looking at dual-channel speeds which Telstra offers around the CBD. But maybe the very latest smartphones support dual-channel and definitely if they use 4G then you should be good with either a phone or a USB modem.