I too have a Howard 350, which I bought new in 1970. As it's getting on in years I bought the Mantis four stroke when we came to live here in France and started buidling a veg garden out of a field.
I find the Mantis is very slow. Any stone the size of a pigeon egg will likely jam the rotors.
The spread of the tines leaves a central ridge that requires going over again using a diagonal motion or criss crossing the plot from a 90 degree angle from the first pass direction.
Walking backwards takes a little getting used to - I would prefer a swivel to offset the control bars allowing one to walk forwards alongside the machine while working.
On hard ground it will bounce around and needs some grip to control it. Another factor of its lightweight.
The Honda engine is good, it uses a lot of fuel to till a plot- combination of slow speed, number of passes required ( more than one) and the 9 inches width of cultivation.
There is a lot of vibration and the lack of self locking nuts means one must check them for tightness before using the machine. I lost the stand bolts on the plot through being negligent of this.
I had a problem with escape of oil into the cylinder while trying to clear a jam, by stupidly turning it over for a moment. The lightness of the machine will allow this to happen.
I still use the machine for but for any serious work I break out the Howard, even although it is difficult to manouvre around the beds I have laid out.
My usual practise is to run forwards over the line to be cultivated then run backwards. This leaves two pronounced ridges either side of the run. One of which is run over on the forward run to start the next row.
For quite small jobs I will often resort to hand digging. I have just hand dug a bed that
grew sweetcorn and squash this year and tomatoes the year before. The bed is 1.6m x
12m. It had been dug previously with the Mantis, so was faairly easy digging, but with the high water content of the soil and digging in manure I knew the Mantis would not handle it. I found it chokes up on manures so digging in is difficult.
The Mantis is expensive for what you get for the money. There are better options.
I did not find the support service helpful at all - perhaps that's as a result of buying in Uk and moving to France.
I have said a lot on the negative side, but its game little machine. It gives agood tilth and goes down 10 inches - which is more than the Howard. It's a great concept, but I feel it is not finished and would like to see some more design put into it.
The tines need to overlap. Perhaps some refillable water ballast tanks to give more weight ove the tines to reduce bounce. An offset swivel perhaps. For weeding between rows, it's good, but why buy a machine for that!
By the way, this is only my opinion as a user and is not authoritative. If buying again I would seriously ask myself exactly what it is I want in a machine and test drive a few before making my purchase to see i get what I want and not let myself be seduced again by all of the very wholesome opinions of other users who find it ideal for their purposes.
Good Luck with your search for a machine.
Hobnails