So I've already searched the forum extensively on battery upgrades...
I am looking to upgrade from the stock 27 Panasonic to a 31 or a 34 battery (Odyssey) and I am curious on some opinions...

The truck is 6 months old and I am slowly building it up to be a surfing exploration adventure rig, no rock crawling more overlanding type stuff. I won't be adding a winch and I plan on keeping stock bumper, but I will be adding lights and some electronics and upgrading the suspension...

31 Pros - Really High CCA and power
31 Cons - Weight, Mild maintenance, battery cable modifications to fit, bigger battery tray

34 Pros - Less weight, No battery cable modifications to fit, No battery tray
34 Cons - Less CCA and power then the 31 (but still better than the 27 stock)

I guess what I am asking here is, money aside, is the weight of the 31 worth the gains over the 34 on a rig for mild adventuring.

Everyone's Mileage May Vary...
but anyone with some experience with these in their truck goes further then my googled knowledge...

I am still on the OEM battery but I think for your needs the 34 would be ideal - I am leaning that direction when the time comes. It really does depend on if you need the extra capacity but the reality is if you do you can always get a solar charger, run the truck for a while, or have a small Honda generator (pick you favorite brand)

I've got a group 34 Odyssey. No issues, obviously, but I've never run it down so I couldn't tell you if it's got enough juice for you.

Just make sure you get the spacer for it. It's like a $4 part from Odyssey that makes the new battery tall enough to work with the factory tie-down.

For what it's worth, I went with the Group 31M. For less than $50 for a battery tray and a couple of cable extensions it pretty much drops right in. (At least in my 2011 Trail it did) A few bucks to fix up the battery tie down and you're good to go.

If your at all worried about running out of juice, I would go with the 31M just because you can......

At what point does it become necessary (or at least wise) to upgrade the battery and/or alternator?

I don't need a winch, but I would like to add some lights eventually. And I currently have a lot of electronic gadgets (radar detector, dash cam) that either operate off the 12v outlets, or have batteries that I'm frequently recharging via the outlets (several cameras, drone batteries, iPhone, etc).

You covered most of the pros and cons in your first post. Ideally if you want to go with the largest battery you can keep in the stock location to keep up with power requirements etc. A winch, fridge, air compressor has a huge impact on draw. A group 31 has about double the capacity of a 35, and a marginal bump from a 34.

That being said, I've had issues with the sheet metal cracking from underneath the battery location. I welded it up the first time, now it's cracked again in two different locations. I may end up with two smaller batteries to better distribute the weight.

I know you are asking for 31 or 34 but you might want to consider 24 series (it also fits with stock hold-down bracket:

If all you are doing is adding lights and stuff just stay 24

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I haven't had those issues, but I know you've run your rig pretty intensely over some crappy terrain.

Victor, what tray were you using? I found the Pelfrey cage eliminated any hopping or vibration caused by such a big battery. I'm not saying it would have eliminated your cracking, but it might have staved it off.

I run 2 group 31s, one in front, and one in the cab. But I'm running a big inverter and a few other things inside for work and camping (fridge, etc).

I just finished repairing mine for the 3rd time. I'm installing that battery cage this time to see if it makes a difference.

I also have a strut that connects to the radiator support, but not sure if I'm going to connect it just yet. It seemed to have made the cracking worse when I connected to the top of the stock tie down.

I had an aftermarket steel battery bottom tray, and no matter what I did the battery would twist and torque around no matter how tight i had the battery strapped down.

The Pelfrey cage just doesn't go anywhere. I am not saying that would eliminate the issue, but I think it'd help.

Does the pelfreybilt cage bolt down? It looks like it just uses the factory j hook locations

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It uses J hooks but it uses huge and beefier j hooks.

you can buy beefy j hooks for $5

There's more to the design than just bigger hooks, and I apologize if I'm not describing it in detail. I wish I'd taken photos during the install at Goose Gear last October, but the cage itself is much more substantial and just a strong top tie down.

It certainly helps keep the Group 31 in place.