In terms of quality, for the same cost, you will get a better boxlock than you will a sidelock. It should be remembered that both actions were made in a wide range of qualities. I have seen some awful sidelocks made at the cheapest of costs - notably the BRNO "sidelock" of the 1970s/80s - and some very high quality boxlocks - notably Churchill's "Utility" and Greener's "Empire", or as I have at the back of my cabinet a 16 bore self-opening French Robust side-by-side boxlock.

Reliability is an issue that goes "hand-in-hand" with repairability! Easiest to repair, in that it is easiest to disassemble, is the sidelock. Two screws and it all comes off. If you need to look to see what is broken...and what it should look like when not broken...just take off the other lock!

Although in theory the boxlock should be easier to repair as it has fewer parts. The problem is getting at them in that to repair it the action must be removed from the woodwork.

In the wet, again, in theory a sidelock should be easier to "dry out". Just take the lock plates off.

Having said that for Ł1,000 or whatever sum, unless you get very lucky, you'll get a better boxlock than you will a sidelock. But there is more to it than that. It also then is necessary to cosider what sort of lever work you have, if you have a "top extension" or "third bite" (an abomination in any standard 2 1/2" or 2 3/4" 12 bore IMHO) and quality of woodwork etc., etc.

A top quality sidelock will if in good order be far "better" than a cheap boxlock. And it is on that that many people say whether one is better or not. That is wrong. For a top quality boxlock will be better than any cheap sidelock.

My advice? Get the most gun you can for the money you have and don't be "hung-up" on the sidelock vs boxlock dogmas. If your budget is Ł20,000 choose a top London, or Birmingham, sidelock. If your budget is Ł2,000 choose a top London or Birmingham boxlock.