I've used both for edging. I love the smell of a lot of alyssum, and they seem to need less water for me. Also, they set off the color of most other flowers, whereas lobelia can kind of blend in. For me, lobelia seems to make a much larger mass. I've had to shear alyssum, but never lobelia.
For this year, you should try some of both; then you'll know which one you prefer for next year.

I edge my front bed with lobelia and love the intense blue purple. It blooms from june through first hard frost (it will last through light ones). I used to buy two flats of it for this, but grew it from seed this year, and saved 30 dollars or so. Alyssum always ends up scraggy looking for me.

I gave up on Alyssum years ago in Ohio because it would not recover if it ever wilts, which it always seemed to do, even when other plants near it were still fine. It's also hard to get a thick enough mass to look right. I like the suggestion to try both, though. You might also like portulaca, moss roses, which dry out less often once established than both of these.

Funny how different plants behave so differently in different places. Alyssum never wilts for me, and lobelia wilts several times every summer. I use lobelia as my "oh gosh, I'd better water everything" reminder!

I use both alyssum and lobelia in my flower beds. Lobelia will spread and bloom all summer if in part shade. Alyssum spreads and blooms heavily for me in full sun. However, I found that the white alyssum spreads and fills in an area much quicker than the colored types. It is a staple in my garden because it spreads into a big, fluffy white mass, blooms heavily all season and the fragrance is so light and sweet. The white goes with any color combination I do, I've got it in a bed of marigolds and rudbeckia, in another bed of pinks and purples and in pots of various color combinations. I have never had it wilt, however the lobelia will wilt for me if it is in a sunny spot. Lobelia is another staple in my shady beds, I use the baby blue and sapphire blues with impatiens and begonias in pink and white. In pots I have the sapphire blue with yellow violas and the color combination is beautiful. I think both lobelia and alyssum are super easy to grow as long as you put them in the right spot. Another big plus for me is that wildlife does not seem to eat either one.

purpleinopp, re the alyssum in ohio. I used to plant the white every year in beds and it did wonderfully, never had to fuss over it. The last three years or so, it sits and doesn't fill out and also will wilt. I wonder what has happened? Maybe nurseries are offering a less vigorous variety now or something.

The only white I've grown is Snow Crystals, this variety has been around a long time and is fool proof for me. Heavy bloomer, no fuss, fragrant edger and container plant. Its very easy to winter sow, too. I bought 2 packs for $.20 each and got 40 plants out of it. Not bad, and generally, my winter sown seedlings are hardier and healthier than the plants I purchase from local nurseries.