My ancient Silva has got a bubble in the fluid, and is getting slow to react, so it's going in the bin and I'm after a new one.

The only difference between the cheap £10 Silva and the next one up seems to be the size of the baseplate - is there any reason why I need to spend an extra £10 to get a bigger bit of plastic?

My old one was the smallest, and it's always seemed ok to me, but maybe I'm missing something...

If you've only got a small baseplate it increases the chances that if you're taking a bearing off the map then the edge won't be long enough to accommodate both ends of the line, so you'll have to do some guessing.  Chances of that go up with larger scale maps.

But if you've never found that to be a problem then I wouldn't worry about it!

Pete. 

I used to kick around the hills with a pretty big compass clinometer, which had a fold-out mirror so that you took bearings using the 'sighting' method. I was doing serious geology at the time, which is why I got that model. When I switched to a 'normal' compass, I got the smallest Silva model, and I'm still using it years down the line. I can't say my navigation has suffered in the slightest, and I never seem to need to take long bearings, so it's fine for me. If you've been doing the same, then stick with the small one.

Now... if I ever turned my hand to instructing people in the use of a map and compass, I'd invest in the biggest, simplest compass I could get my hands on, with big, bold print for the cardinal points and numbers. I've always thought it pretty pointless when I've seen someone trying to instruct a group, especially kids, while using a tiny compass that's completely lost in their huge maulers. I think Silva provide a huge 'dummy' compass for instruction, but of course the arrow isn't magnetic on those, since it's just a visual aid.

Yes there is a reason - I think expedition 4 is more than just larger. 

It has luminous dots on the base plate and inside the bezel as well as on the pointer.  This can aid navigation at night.  It also has rubber feet so it will not slide off the map.  1:25000, 1:63000, 1:50000 romer scales are printed on the base plate.  It's a good compass for reasonably acurate navigation. 

I carry my mum's old (as in, decades old) series 7 or my Cammenga lensatic compass (depending on what I'm up to), and a Silva protractor.  The protractor is mostly used for the rhomers, but basically it's a cheap, light, flexible base plate.

Series 7 + protractor = £10 + £1.50 = £11.50

Expedition 4  =  £30

The only advantage of using a baseplate instead of a protractor is that you don't actually need to use the numbers around the compass bezel.  You can use bearings entirely mechanically.  When sighting, though, you *do* rather want to use the numbers.

If you want a longer one for the same price the Recta DT100 is longer than the Silva, and has 1:10k,25k&50k scales on it as well.

 http://www.cave-crag.co.uk/ProductDetails/mcs/productID/1329 

Is the bubble as big as the one in mine, seen here demonstrating magnetic problems on Mull.