Hi Dave,
I'll try to answer your question from my experience of both these locos. Firstly have a look at my YouTube videos which are posted on the Mamod website -
http://youtu.be/aTH7_uuTiuc &
http://youtu.be/h_E_0eGpgAk The Saddle Tank is running with a Summerlands Chuffer fitted in that video.
These are both toy models which are constructed using Mamod's current method of rivets (we have asked them several times to produce a kit version but no progress so far). They use the same boiler, cab & lubricator, and powered chassis with the differences just being the Side Tanks or Saddle Tank. In my opinion the method used to mount the Saddle Tank is worse than the Side Tank, with my Saddle Tank's cab poorly fitted.
Mike advised us of a design fault with the Mamod gas burners, which will still affect the Saddle Tank loco (they were made in a limited batch of 100 - new burners should be suitably modified by Mamod for the Mark II). I've not read the Garden Rail article that Doug (
dougrail) refers to, but looking at the photo in the viewable index on-line the reviewer has been turning up the gas too much (the metalled label has been blackened on the Saddle Tank). I recommend to all new owners that they paint/mark a line on the regulator & gas valve wheels so you know where they are being set to.
We have already discussed the deficiencies of the lubricator design in this Forum, as it is a total displacement design (no through steam pipe) and Mamod have "cheap'd" out on the exhaust pipe (it does not reach up into the chimney). I have found by reducing how much steam oil I put in the lubricator (not as much as the instructions) I get less of a mess on the front smokebox apron, and I've fitted a deflector plate so it does not affect the burner as much.
As to running I've found my Saddle Tank to be the best runner, but it was bought 2nd hand having been run by Mamod on their demonstration video, then running in the garden by it's previous owner. His videos showed that the 'O' ring piston rod glands do leak occasionally (another Mamod design fault) but most owners just seem to accept it (
steamie1's modified Mark II did it on video as well) as it does not really affect the running.
You would need to check with Mamod but they were recently producing the Mark II with an extra boiler insert fitted with a blanking plug, allowing another fitting like a water top up valve or whistle to be fitted. However Dream Steam seemed to be selling the older stock without that extra boiler insert.
In terms of Mamodification the Saddle Tank probably offers the easier option to de-rivet the loco, as the Side Tank's are spot/tack welded to the cab side. However it does have that ugly Diamond Jubilee plaque riveted (poorly on my example) to the cab roof. Back in April 2013 David Terry (Mamod MD) told me they had sold 70 of the 100 Saddle Tanks and you could still get the optional matching ruby red tender.
You asked about the heat over time. So far I've been lucky with my Mark I & II locos where the label on the side tank is just sticky plastic but have not melted yet, but unlike Mike I have not run my locos outdoors yet. I would recommend taking those plastic stickers off straight away as when they do melt (like on my William II) they do leave an imprint in the paintwork that cannot be removed easily. The labels on the Saddle Tank are metal backed, however the bottom of my Saddle Tank already has scorching marks from the previous owner turning the gas up too much or running the loco a lot backwards (the burner gets affected by air being pushed through the cab front which is not fixed like the Side Tank version). I've drilled out the burner venturi and sealed the jet holder in with PTFE tape which has improved the burner's stability. Certainly as you would expect the Saddle Tank loco runs hotter than the Side Tank . I'm not sure what paint Mamod actually use but they do not use any primer first and the black chassis paint is easily scorched by stray flames so I guess it is not VHT paint. The crinkle paint on the boilers seems to stand up to the heat better, but again no primer has been used first.
So it really depends on what you intend doing with your loco. Keeping it in original format but adding a few beautification bits & pieces, or going for a completely new style bodywork. Effectively they are the same running chassis with different tank styles fitted, and you pay an extra £31 for the ruby red colour & a limited edition plaque.
Chris Cairns.