As the owner of a peptide company here in the UK, who is about to start selling SARMS (I’m just being transparent, not trying to pimp), I do feel somewhat qualified to answer your question lol.
So firstly, mk677 is neither a SARM, nor a peptide. It is an oral GH secretagogue, showing potent and long-lasting (24 hour) Gh and IGF1 increasing effects. Common side-effects include significant hunger increases and lethargy. I would say that roughly 80% of users report one or both of these side-effects.
Since mk677 doesn’t cause a GH pulse but rather a bleed (which is not physiologically desirable from a long-term perspective), it should not be run for longer than 6-8 weeks.
I personally have no experience of ostarine, but have interviewed a few users whose common consensus is that ostarine can offer a very mild steroid-like effect on strength and muscle gain.
Ligandrol (LGD) should offer much the same effects as the ostarine. I just did a quick review of asome pubmed studies for you and it seems that although you will suppress test, SHBG and HDL, you won’t suppress LH or FSH, meaning a decent recovery post cycle.
People eulogise about the dropper versions, but honestly (as someone who will be producing SARMS and similar soon), the chief advantage is in the ease of manufacturing liquids over capsules.