My ds is 16 months and still does a bit of both - like you said mostly walks at home but at mums and tots he crawls. Two reasons i think is mums and tots is a big open space with lots of kids running round (he has been bumped a few times) so i think he feels a bit vulnerable walking there, second he can get places faster by crawling. I think its just one of those things that happens slowly over time x
Sounds v normal! They just do whatever they prefer I think. DS bum shuffles, he's 14 months too, only just starting to be able to stand whilst holding onto something, can't pull up, and can't stand alone or walk.
Nursery worker said they'd only worry when he wasn't moving on his own (ie walking or crawling or bum shuffling) by 16 months!
I suspect this is going to be one of those 'every kid is different' - mine took his first independent steps one day, and 2 weeks later had stopped crawling entirely... he's very nosy though and has always been obsessed with seeing what's on top of tables.