I'd say slightly too big too as you might be able to make it smaller if needed but doubt you would be able to mske one larger if needed if that makes sense!
I'd go with the small pair too because it would motivate me to diet (I've actually got about 4 pairs of trousers though that i've done this with and never wear as i skip the dieting bit!)
For the halter though, i'd go with the larger one!