Yes, that 21% extra sure makes a difference. In fact it is amazing just how much of a difference it makes- during a couple of cloudy spells this year (in one now) I've been quite surprised at how quickly I get back to 100% State of Charge in marginal charging conditions, and not needing to run a generator (but would have had to with Lead-acid) is a big bonus
Whether it is equal to 21% more panels is not so clear cut- it depends somewhat on your usage patterns- day-night, how big a load, etc, and I think in some cases it may effectively be better than that. Because you can charge at a very high rate right up to over 95% SOC, you can make much better use of marginal charging conditions like brief sunny breaks between clouds as the weather clears, whereas with Lead-Acid batteries you are stuck with the slow process of absorb stage charging, which needs to run ~3 hours with this size battery for my typical usage patterns. Having a Lithium battery also allows you to better utilise a larger array of PV panels, with no real restriction on how fast you can charge (or discharge) them for typical off-grid systems.
Panels are getting relatively cheaper these days, but you dont need as large a battery if you have Lithium, so it is a bit hard to exacly compare. However, the 800AH @26V of Lithium cells (21kWh storage) and battery balancing equipment I've just bought cost a bit over $10000, including freight.
These 900AH Raylite batteries: http://www.apolloenergy.com.au/Renewabl ... 00ah-p-256
would be a bit over $6000 + fittings and freight, but if you wanted them to last a similar time to the claimed life of my Lithium cells, you could only use about half as much of their capacity, or to have the same effective kWh storage, you would need twice as many of them. Clearly, for a new system, a Lithium battery is the best way to go right now.
They do need a bit more work for the initial set up, but after that, they require less maintenance than a Lead-acid battery, and you dont have the worry about handling acid!
The Outback Flexmax charge controllers are able to charge the LiFePO4 cells, as they have a range of adjustments that covers what is needed for these cells, so there is no extra expense involved. The Latronics inverter also works well with these cells, the only adjustment I altered was the low battery disconnect, which I raised from 20V to 24V via internal switches. This is the primary low voltage disconnect to protect the batteries from being discharged too deeply. The Battery Control Unit also has a LVD, which is there as a backup.