Delphiniums are beautiful and great for bees, but they are not totally robust, as some people think - they do need a bit of tender loving care, unlike many perennials you can't just plant and forget them. keep the blood, fish & bone handy. They love well-drained soil, but are hungry feeders. This means adding plenty of grit on heavy soils to help with soil drainage. If delphiniums sit wet in their dormant winter season, they can die, so make sure you prevent this. I've had that problem with them over the years - too much wet in clayish soil in my neck of the woods. The best results I've had is in raised beds that drain well.
You say you're clearing fir trees to make room for them so you should have a well drained spot where the tree roots have been, but I guess the soil will be impoverished, so bear that in mind.
Personally I like seeds BUT plugs and especially plants are sometimes a better (but a more expensive) option. If they like their spot and you feed them well you should have years of enjoyment from them.