Both types of residencies are competitive, but will weigh different factors differently. A 6-year OMS residency will look at your undergraduate transcript(s) and the NBDE Part 1 scores more heavily while a 4-year residency may not care so much on those factors. They both place a heavy emphasis on Class ranking and cumulative dental GPA.
Hospital and school-affliated OMS residencies are both competitive. While school-affliated residencies are more known and popular in general, but hospital OMS residencies offer less number of spots/seats. For example, a hospital OMS residency may only accept one resident per application cycle while a school-affliated OMS residency may accept 2 or 3 per year.
Bottom line, whether it's a 4 or 6 year residency OR hospital or school-affliated OMS residency, they're all just as competitive as one another. Obviously some are more than others, but usually it's the tradition, history and the name of the school/hospital (ie. Mayo, Harvard, UCLA, etc.)
You, as an applicant, can't say that a 4-year, hospital based OMS residency is less competitive than a 6-year, school-affliated OMS residency! Every and each OMS residency is a bit different, some have more training in one aspect of OMS than others and some have more recognition/name than others.
I would say big name residencies are more competitive than less popular/unheard of residencies, but competitiveness is not based on the length of the training or either you're at a hospital or school-affliated OMS residency.