The Blue Clips campanula (campanula carpatica), which is also available in White Clips, is one of my favourite plants. It grows into a mound no more than about 8" high but given good soil, adequate moisture and at least half day sun, it will have lots of blooms over many weeks. It's hardy to zone 4 but I have lost some plants if the spring or fall is overly wet. Not a problem as they are readily available in the spring at garden centers for a modest price. Many seed catalogues also offer the seeds and they are pretty easy to get going and will bloom the first year after seeding if you get them started early enough. They make a great front of the border edging plant.
I haven't seen the peach leaf campanula (campanula persicifolia) at the local nurseries. I bought mine from Rockwood Gardens through the mail a couple of years ago. I'm really happy with it. It is a spreader but I have it where it has room to spread. I haven't watered it all summer and even though we've hardly had a drop of rain all July, it is doing just fine. It blooms in June but still has the odd flower now. The flowers come out along the stem which grows to about 2' high. It doesn't flop over because the stems are very stiff. It's a lovely mauve colour and I have paired it with a scabiosa of similar colour. Definitely hardy to zone 4. It enjoys lots of sun.