My study of magic and metaphysical healing has emphasized magical herbalism from the beginning. The first pagan book I bought for myself was Cunningham's Encyclopedia of Magical Herbs. I knew nothing about plants at the time; I couldn't even identify lavender or rosemary, and I was a little shaky on dandelions. Sometimes figuring out which herbs to use in a spell was very difficult.

I hadn't yet learned to hear my intuition, much less trust it. So I usually chose herbs off the lists of correspondences in the back of the Encyclopedia and hoped I could buy them from the botánica in my New Orleans neighborhood, or from the bulk section of the Whole Foods across town.

Mind you, I think choosing herbs according to availability is a perfectly acceptable practice. Think about it: Cunningham's Encyclopedia lists about 54 herbs under "healing, to promote". Looking at this list, I see adder's tongue (Erythronium americanum) listed first. I'm not familiar with this plant--it grows on the other side of the U.S.--and it's definitely not available from my favorite herbal supplier. But further down the list is blackberry (Rubus spp.). Blackberries are weeds in Oregon: I have some at the edge of my garden.

In my experience, plants don't work better just because they're harder to get. In fact, I'd take a fresh, vibrant plantain leaf over a dry, dusty, ten year old saffron strand any day--especially for healing magic. Healing calls for juicy, lively energy. There's nothing more exuberantly alive than a weed.

Another crucial decision-making factor for me is the environmental impact of growing, harvesting and distributing the plants in question. I'm not comfortable doing a work of healing magic if the price of my spell is to deplete the populations of rare or endangered plants, damage fragile eco-systems or to exploit humans and/or animals. So I work with plants that thrive in my back yard or at the roadside as often as possible. This practice has the added benefit of reducing the amount of fossil fuels used to acquire my magical supplies, and of giving me the assurance that the plants I use are of high quality.

As I've developed relationships with many plants, I've come to understand more about the nuances of how they work. So I sometimes consider the effect of a plant on the physical body as an indicator of how it might work at the energetic level. This method requires either experience with plants, or a willingness to spend some time learning about them, but it can be very helpful in getting just the right energy for your working. 
It's also best used when you've narrowed your choice down to two or three plants, but for whatever reason want to pick just one.

For example, let's look at the properties of comfrey (Symphytum spp.), and marigold/calendula (Calendula officinalis). Interesting aside: neither of these plants is listed in Cunningham as plants of magical healing. Perhaps they aren't traditional choices, but they're at the top of my list for both physical and metaphysical healing. You may decide you don't like my choice of plants at all, and I completely respect that. For me, magic is a wonderful combination of established tradition and here-and-now, ask-the-plants improvisation. Now, on with my example.

Imagine you're crafting a spell to heal your coven after some internal strife, and you're deciding between comfrey and calendula. Maybe you're are on a tight budget and can only afford one. Or maybe your sources for the two plants are on opposite sides of town, and you don't have time for all that running around. Whatever the reason, here is where knowing a little bit about the nature of each plant is very helpful.

About thirty seconds of research will show you that calendula is used to treat burns, cuts, and skin irritations. The word "soothing" is often used in descriptions of calendula. Now Google comfrey, and one of the first results will be the following: "Comfrey has been found to cause cells to divide at an increased rate, thus healing bones and wounds more quickly." In fact, comfrey's acceleration of healing is so pronounced that many herbalists warn against using it on wounds that haven't been thoroughly cleaned, as it can seal in infection or foreign objects.

So how would you interpret this information in the context of magical healing? In this case, you might consider whether or not the poison has been removed from the wounds of your community. That is, have the issues separating community members been resolved, or is vitriol and gossip still spreading like an infection? I would choose comfrey for a healing spell only after deep work has been done to remove the source of discord, and the time has come to re-weave my community and recreate close bonds between members.

Calendula, on the other hand, is gentle enough to use on babies and animals. It has disinfectant properties as well as soothing properties. In a case where disharmony is still plaguing your community, calendula could be called on to sooth the irritations and wounds, and facilitate a clearing of the causes of the trouble. I would choose calendula when the primary need is to calm the uproar and comfort community members who are hurting, but there is still much work to be done to remove the source(s) of strife.

Obviously, this is just one method of interpreting the nuances of different plants with similar magical uses. Your mileage may vary. In the end, what matters most is finding the plant that resonates most strongly with the kind of energy you want to call in--and that could be different for every person.