Just conventional, I haven't made the leap to any kind of Command yet, this will be the first Command I have.
Best of luck with your decision. If you don't mind my $0.02, I'm thinking you and your son will outgrow the LC engine quickly. We have a Thomas engine with LC technology, but it's so unsophisticated compared with TMCC or LC Plus let alone Legacy. Not sure how to advise, but $400 isn't chump change, especially when you've stated you don't really need the rolling stock, track, or transformer that goes with the set.
I'd bet that some of the LC Plus engines could make it through the O27 switches. Maybe start a separate topic for owners of LC Plus engines. You might be just fine with, say, an LC+ Mikado. The TMCC Mikados were rated for O27.
I run traditional-sized stuff but with original TMCC and no plans to go to Legacy. I have a few modern conventional engines and am either upgrading them to TMCC or unloading them. Just leaving me with postwar for conventional operations.
If I was starting over, I'd probably go with LC Plus which seems to have replaced TMCC in the traditional-sized market. But, since I've had some TMCC for over 10 years, I can't see replacing them.
The LC Thomas engine we have is nice, but without smoke, crew talk, or electrocoupler, it pales in comparison to the TMCC engines. It's an entry-level toy and not cheap. Original TMCC or newer LC Plus have a lot more toy value IMHO for the $400 you're thinking about spending.
If LC Plus can't run through your switches, you could get a gently used O27 TMCC engine for under $200, maybe less. I've seen my O27 Pennsy Jr. Berk with TMCC go for $150-$200 these days. TMCC pales in comparison to Legacy, but it's better than LC. You'd have to invest probably about $100 on the used market for a CAB-1 and command base, but, again, if you're just looking for an engine, I think you can get the whole TMCC outfit for $300 on the used market (starting with forum members) and get more play value.