Hi,

I have a summit and have used the standard Etrex. So here is MHO:

The extras on the summit are altimeter and mag compass.
The Altimeter is usefull if you intend on doing climbing or hill walking as GPS altitude is usually quite a long way out, if you don't need this info it is useless.

Mag compass - maintains the pointer towards wherever you are going. With the standard model you have to move for the gps to know what direction you are travelling in. This saves the getting to the carpark then heading of in the wrong direction for 20 paces then turning round once the gps knows which way you are going.

In real use I can't say I have seen much advantage in the summit to be honest. The higher level versions provide maps & places of interest which are handy for navigating in the car but as you can't get UK topo maps not that helpfull when walking.

Hope that helps.