Hello everyone! 
We're asked the same question pretty much all day, every day - 'which gasket is best for my Rover / MG, the MLS or Elastomer?'

We have a comprehensive guide to help you choose here: 
Choosing the correct K Series Head Gasket Kit

These days we're much more cautious than we used to be in recommending the MLS gaskets - and here's why. 

I'll start by defending the N Series gasket - this is genuinely a HUGE step forward in design, especially when used as part of the complete kit.
I wholeheartedly recommend these, and will continue to do so - as long as the liner heights and cylinder head are in perfect condition (see the guide above), the N Series kit is a long-term fix that addresses several of the K Series weaknesses. 

The issue with every failure of the MLS gasket I've personally seen is that the liner heights are either not checked at installation (meaning failure is inevitable and quick if one is lower than the rest, or they're all below the height needed to form an effective seal), or the head has fire-ring indentations that haven't been resolved before putting everything back together. 
While I've prattled on about liner heights since I learned to talk (well, not quite) - the issue of poor-quality heads is something I've not really had to touch on until now. 
Recently I've been investigating as many head gasket failures as possible - a recurring theme when speaking to owners is that the head was just cleaned up and refitted, without checking how deep the fire ring indentations are. 

The golden rule is that if you can feel the fire-ring indentations with a fingernail, the head needs to be skimmed (or is scrap, depending on if it's already been skimmed. There's only so much you can remove from a K Series head before it's fit only for the bin). 

Fire-ring indentations are seemingly a weak point for the MLS gaskets - while the original Elastomer design uses separately fitted fire rings that are quite thick, the MLS does not. This unfortunately means in cases where the head does have quite severe fire-ring indentations, the MLS gasket is much more likely to fail. 

Recent changes to the Elastomer gasket also mean this is now much more reliable - our version of the Payen BW750 uses an Elastomer beading that is designed to be more resilient to heat, especially on the exhaust side of the head where temperatures get uncomfortably close to the breakdown temperature of the original material.
Just a small amount of coolant loss is all that's needed to take the temperature above the breakdown point - leading to oil and water mixing and a sump full of Hellman's mayonnaise. 

I also believe a large amount of the MLS hype is due to the original design suffering with such widely-publicised issues - any gasket that promises to be a big step forward for such a problematic engine was bound to attract a loyal following.
When released, it was touted as a fix-all solution to the K Series - an engine lamented for many years by the public and motoring press. While it is indeed much stronger at face-value - not relying on soft elastomeric beading as a sealing medium - in practice the MLS vs Elastomer pros and cons list is nowhere near that simple. 

Finally, a well-known MGR specialist has recently noted several cases where cheaper MLS gaskets have corroded away to nothing - this issue doesn't seem to affect the Elastomer gaskets, being of a coated / stainless steel construction (depending on manufacturer). 

I hope you find this guide useful - we'll of course continue to sell both MLS and Elastomer Head Gasket Kits (see our range here).
This article is purely to convey our thoughts on the various options out there, and our experience with them in practice.