I can't see your first link there but the second one looks really big for everyday use. Have you looked at the basic range slim line butt plug? It's currently in the £5 sale weekend going on atm so would be worth a look.
I've heard it said that plugs or dildos will not stay put, not had the opportunity to do the experiment myself.
Another option to keep a toy in is some rubber panties with the toy built-in, there's a lot out there - have a look in the suppliers list at http://www.rubberist.net, should find loads to choose from.
I tried wearing my butt plug inside me whilst walking to the shops once. But it was SO unconfortable, i had to take it out once i reached the store. I dont know if i did somethign wrong, but it didn;t give me the sensations i was hoping for. Has anyone else had this problem?