Welcome to the party Flumpy.
When it comes to your question/thread title of: Mega/Meka dred vs gork/morkanaut, for me there is only one answer. The MekaDread.
Using the 6th Ed update to IA8, the Meka can be a Warlord and doesn't use up a Heavyslot. Is it harder to get a MekaDread? Possibly. It really depends on if your local meta is down with Scratchbuilt Konversions. A fair deal of opponents I've had in the past (and occasionally still show up at the stores I frequent) have been opposed to scratchbuilds and conversions, but usually that's due to an experience with someone who modeled specifically to gain a tactical advantage, something that I'm personally against.
When GW brought in 'True Line of Sight' I was afraid some of my opponents were going to rally against my scratchbuilt minis, and while some did, most saw the two important tenants I follow when it comes to scratchbuilding. 1 that the scratchbuild adheres to the correct size of the official mini; 2 that it looks appropriate. Cover both of those and your Gold.
If you think you can scratchbuild a good Mekadread, then I'll be the first to say Waaagh On, and can I see some WiP pics. However since many players, particularly newer ones are less inclined to put in the hours (and I mean HOURS) of
challenging work to scratchbuild, I'll say this; if you are more interested in the gaming aspect of the hobby than the modeling aspect (which I totally understand and respect), go for the Morkanought instead.
The Morkanought is a different beast than the MekaDread, it just is. But the nought is a perfectly valid choice, and will never require Forgeworld or extensive modification (although AoD's light up is quite amazing, and not to toot my own horn but I rather dig my own Scratchbuilt Morkanought which you can see here