I have converted all of my Ryobi One+ shop tools to Li-ion batteries and really like them vs NiCad. I am not aware of any Ryobi tools that are packaged with the 5-6 Ah battteries; seems like all tools are packaged with the small batteries. However, HD does seem to offer some 'buy-it-together' combo deals... Not sure how much of a deal it really is - - still looks like $99 for a 'big' battery.
Unless you have a very small yard, I'd recommend you stay away from the One+ line of lawn tools. The service factor on a cordless drill is maybe 0.1 or 0.2, compared to a line trimmer with a service factor of 0.8-0.9 (over any given battery's one-time charge). As stated above, my contention is that this high SF is just murder on the 18V batteries.
If you have more than a postage stamp yard and want cordless lawn tools, I'd look at the >40V units.