From my own experience, the LRC is an organisation that is quite small in number, and dominated by middle-aged men. What's more, they're brimming with dynamism compared to much of the Labour left I briefly got involved with, which felt moribund and decrepit at times. They may have more political influence than the SP, but didn't feel like it was a group on the up.
By comparison the SP branches I've come in contact with are much younger, less homogenous and less male dominated. They may be totally politcally marginalised, but the average SP branch meeting has more people, and more interesting discussion, than the typical labour party ward meeting. The problem they have tho, is keeping them involved, not attracting them in the first place. That's true of the SWP too, moreso infact, they have this revolving door where they sign up a few thousand freshers every year at red brick uni's, then they all leave and they sign up next year's freshers to replace them, ad infinitum.
Also, when I speak to ex Militant people who were in Labour, not all of which are currently in the SP btw, the consensus is that there's no audience at all for left wing idea's in the Labour party, that in many area's it's structures are hollow and decrepit, and that many of the attractive features that entryism once had simply aren't there any more. The Labour party isn't where the class is anymore, and joining Labour holds less appeal to sincere left-wingers than at any point in it's history.
It's also not a place you'd go to recruit people into a radical group. Thinking about some of the people who go to SP meetings, and anarchist meetings, that I've been too, most of the people there wouldn't really define themselves ideologically as either Trotskyites and Anarcho-syndicalists, and I have often thought that maybe a generation or two ago a lot of those people would've gravitated initially towards Labour party young socialists, and then perhaps become more radical later on. Today though, no-one with any nascent radical inclinations would think to join Labour, it seems totally inappropriate.