I suggest you dig deeper into the problem.
I have used both the above batteries in my 6.5's. After I bounced the truck off an elk and blew the top off an one of the two Optima batteries I was still able to start the engine with them after cutting the fan belt off. The crushed Optima held it's voltage for 2 weeks before drying out after I removed it. These are pretty tough batteries compared to their corrosive acid every where and over everything competitors. Since Jonson Controls bought them out and sold out your fellow Americans to Mexico the quality has gone downhill. They are still better than the flooded cell batteries as I have only ever had one leak on me and it was 6 years old - Made in Mexico. I have been using the brand since they came on the market and am only now seriously looking at their competitors. Odds are nearly 50/50 for the folded cell stuff leaking at the posts and eating the cables. AGM is the way to go and there are better brands out there now. In summery you do have a very good battery to start with. You will pay more for an AGM that is better than an Optima.
So test your batteries. Pop the hood and take a voltage reading. You need 12.8v to indicate a fill charge. 12.4v is discharged somewhat or old batteries. Charge them up and let them sit an hour and take another voltage reading. Then disconnect the batteries and check with an ammeter for a large current that may be draining the batteries. Like glovebox light, dome lights staying on, aftermarket radio or amp not powering off...
What is the load test on the batteries, disconnected from each other telling you? If the load test says the batteries are good - your problem is elsewhere. Spend your money on fixing the problem and get batteries you like better later. If the load test is bad - follow the special charging procedure for an Optima battery. Check other things like a bad charge wire from the alternator. What is the date of manufacture of the batteries. Call Optima with the serial number and they will tell you if it isn't on a sticker. In summery be sure a battery is bad and spend some time asking why to make sure something isn't causing them to go bad on you.
If you have 12.8v before you hit the starter and you are not getting 100 RPM cold you do have an issue. Just because you replaced the starter doesn't mean they did their job and gave you a WORKING starter. So you need to "start over" and recheck everything. For example I had a nice rebuilt starter, good bearings and bushings. Fresh looking brushes. Spotless copper bars. And it literally ground over slowly. Opening it up the starter smelled like burning ozone. A close look found one of the insulators was shorting and burning taking 1/2 the starter's power with it. PO had it done so no warranty. A local auto electric shop that rebuilds starters and alternators is a great resource to make sure the starter you have is good or supply you with a locally rebuilt unit. Chain parts stores are a grade below a local shop and cost more!
However you need to check the battery cables. Simply replace the $20.00 worth of ground cables to the batteries and give the positive cables a once over. Acid wicks down the cables and corrodes them from the inside out. If the cables are stiff they are toast. Seriously, Positive cables do go bad and require replacement esp after a battery was leaking at the positive connection.
2) Optima Redtop 75/25-925 with 910CA / 720CCA / 90 min reserve / Dual post for $189.99 ea , what they should have sold me was this: (2) Optima Redtop 34/78-980 with 1000CA / 800CCA / 100 min reserve Dual post for $189.99 each ....THE SAME PRICE!
