Blueangel, you must not have any little children or pets..LOL..agave..once you have it, you have it forever. I've cut down mine,can't tell you how many times and it still grows back. In pots, they are wonderful and less dangerous. I tried snipping off the pointy tips but they didn't like that.
Love dahlias.

I do have
2 Mini dachshunds so maybe if I go with agave I
will in containers.

Blueangel

The problem with the tree dahlias is that their blooms are way up there and hard to see well. You can keep them pruned to a decent height but most people grow them for the extra tall size. And they bloom really late so some years if we have an early frost you may not see any flowers. Lots of breeders are working with this species so they may have improved this factor.

If you want an agave look without the 'ouch' you can grow that hybrid agave/manfreda (or Mangave) 'Macho Mocha'. Not truly winter hardy here but with a tiny bit of protection it will survive most winters. I like the way it looks, like an agave with purplish speckles - the more sun the more spots. Can get around 3 feet across and 2+ feet tall. It takes a few years to get blooming size and it dies after it blooms but makes a lot of babies afterwards.

Most of the Agave paryi (I'm guessing at the spelling) point their spiney leaf tips up so they tend to not stab anyone or anything walking nearby.

Personally I haven't found Agaves to be very easy to grow here. Now Yuccas are dead easy and come in a wide range of species with a few named cultivars. I would start very early in the season with those dahlias. I lost Dahlia imperialis once and haven't tried a second time.