I've decided to break down and get an Optima battery for my 73Z. There are two sizes that will fit. The 25 and the 34. What do you guys think would be the best one to get?

Don't waste your $ on a $$$ batttery like an Optima. Fix your electrical problems, whatever they are instead. A middle of the road Interstate battery should last you 7-10 years provided you top it up with water once a year. Trickle charge it if you garage your machine for the winter. And repair whatever parasitic draw or charging problem you have that's even making you CONSIDER buying such a fancy battery.

From my experience, I would only get an optima for a daily driver. I still have one that is more than 12 yrs old. Of course this was when they were still made in englewood and not in mexico.

The main reason I bought them was because the dry cell. But too many discharges caught up with the battery I had in the z.

At the time I bought 2, one for my various daily drivers and one for the z. The one in the z only lasted 6 yrs.