I've seen Ostrichsak post on this topic in other threads. He clearly has a bias against Ohmmu for some reason. From my experience, in 4 years I've had the OEM 12V battery replaced twice by Tesla, both times under warranty. That's an average of 2 years each (we'll see how long the 3rd one lasts). Now that my warranty has expired, when I have to replace the battery I'd seriously consider replacing with the Ohmmu unit. The main reason is warranty. Their website shows the warranty as:
"48 months from date of purchase 100% replacement coverage for performance failure of any sort. "
So they guarantee it will last at least 4 years. What's the warranty on the Tesla battery? I haven't been able to find it listed anywhere. But given reports by others having had to replace after 12 months I suspect it's only warranted for 90 days.
Regarding price, my friend has to replace his 12V battery and Tesla quoted him $275 installed. When I last had mine replaced under warranty I was quoted the same, though I didn't have to pay. So you could get lucky and the Tesla OEM may last you more than 2 years, but otherwise, the Ohmmu is more cost effective and will require fewer replacements.
Also keep in mind that the life of the battery depends on how often you drive your car. If you drive frequently the OEM battery will likely last longer since it will rarely require a deep cycle recharge. When your vehicle sits idle in the garage for long periods the 12V battery will run down before it automatically recharges. These long deep recharges are what deplete the battery life on Lead Acid Batteries. Lithium batteries are much better at this and are more likely to have a longer life for people who do not drive their car every day.
Ostrichsak is welcome to his opinion, but it's just that, his opinion.