There is no winning an argument that is based on emotion but I have not had good experiences with the 37 and much prefer the 870 for the following reasons-
The parts thing for example- the last 37 I bought had a timing issue. We traced it to the lifter, want to guess how much THAT COST? I paid only slightly more for the gun if that gives you an indication. Many of the guns I have examined have cracks in the buttstock where it joins the receiver, some worse than others-most useable as is but a few in real need of change out. Replacement wood is in limited availability and expensive for the Ithaca. I can go on Ebay right now and have my choice of 870 stocksets for $25.
I have had MANY occasions where I have wanted more than 5 rounds in a shotgun. Now understand that I am not much of a bird hunter, I dove and quail hunt socially-it seems like a waste of time shooting tiny little critters though. If you want something to eat go shoot something worth shooting- 200lbs and up, covered with hair.....but I digress....
So, my shotguns are primarily directed toward defensive use. In any tactical shotgun course worth it's salt they will run your gun dry- several times a day. It is one of the commonly recognized weaknesses of the shotgun as a tactical weapon-limited capacity and slow to reload. My way of thinking having run a gun dry several times and standing there with "bad guy"-targets still standing, more is better. This is especially true when you store the gun in cruiser ready, your 5 rounds just became 4 wherea with an 870 and an extension I have 6 or half again as many. Now, if tactical shooting is not an issue, none of this applies but for tactical application the 870 wins hands down on this issue and the whole aftermarket accessories department as well. Some of the add ons are junk and I prefer simple guns but some of them solve a real problem.
Barrels, there is exactly ONE m37 barrel on Ebay at the moment. It has 4 days left and it's already at $90. There are 75- 870 barrel items listed, now some of those are for clamps etc....but I can guarantee you that you WILL find the barrel you are looking for be it a .410, 28 gauge or 12 of whatever flavor and the price will be right. I also know that the barrel I buy will go right on the gun, not so with the Ithaca system, barrels are not fully interchangeable. I am in the position of having to provide 10-12 shotguns as loaners for a large Pheasant hunt every year. My 870's spend most of their time with Riot barrels afixed. Pheasant hunt comes and I spend 30 minute changing barrels. Voila! I now have 12 pheasant hunting guns which were an hour before alley sweepers.
And that brings up another issue- How many M37 Magnums have you seen? Yea, I thought so. Very rare and only the newest guns have choke tubes either where with an 870 it's easy to find a tubed barrel.
I just think the 870 is the superior all around gun. That doesn't mean the Ithaca won't get the job done or that a particular Ithaca isn't the better gun for a particular job but overall the 870 due to ease of configuration changes and popularity wins out. On top of that they are as reliable as any other design, proven by the large numbers in service that have digested uncountable numbers of rounds. You know, Remington could still be making the 37, they were in fact the original producers of that Browning Patent. They chose not to.