Be careful what you wish for.

It's an old adage, but a useful one, which the First Minister might do well to consider as she makes her post-Brexit powers pitch to the UK government.

Nicola Sturgeon is demanding Westminster give Holyrood no fewer than 111 responsibilities currently exercised by Brussels.

These powers should come direct to Scotland, not to Whitehall, the First Minister and her ministers say.

But despite the SNP Government's rhetoric of a Westminster 'power grab' Scottish ministers have (slightly less loudly) also made it clear they agree there should be common UK 'frameworks' in some areas.

You never know in a negotiation, but if the precedent of the talks on the fiscal framework set up after Scotland got more tax powers is followed, there is likely to be a deal in the end.

Which means substantial new powers and responsibilities coming to Scotland, and which will be heralded as a great step forward by the SNP.

And it will be, in terms of further powers coming to Holyrood.

Yet with powers come responsibilities and the need to make choices about how to use those powers, including in areas where difficult spending decisions have to be made.

New powers for Holyrood? Credit: PA

One policy area which may be coming to Holyrood - and just to be clear it is still a 'may' at this stage as there has not been agreement - is agricultural support, currently administered under EU regulations.

Let us assume that it does, meaning that the Scottish government is given control over some half a billion pounds worth of spending.

Great say the SNP. We will continue to help farmers but we might also do so in a way which better reflects the needs of Scotland.

Farming north of the Border is different from England and Wales. There are more hill farms, struggling to make ends meet for example, including many in the south of Scotland.

A new regime gives Scottish ministers the chance to shape a new system, to spend the money differently perhaps. Devolution in action. Westminster power grab thwarted. What's not to like?

Well, as academic David Heald pointed out in evidence to Holyrood's finance and constitution committee yesterday, there could be a downside.

If that half a billion comes into the Scottish budget and if there continue to be constraints on public spending - and just about everyone agrees there will be even if they don't like it - then this could cause tensions.

The money once spent on farming, and administered under EU rules with little Scottish discretion, will now be in the general Holyrood coffers.

And as there will inevitably be demands on those coffers for spending on other areas of public services, ministers will be faced with a dilemma.

Prof Heald put the dilemma Ms Sturgeon will then face dramatically: "Do you spend the money on sheep or on nurses?"

Dramatic it may be, but it's a question the First Minister is likely to face.

If Holyrood gets more powers, where would the money be allocated? Credit: Peter Byrne/PA Wire/PA Images

Now, Scottish ministers will undoubtedly say that they can spend money both on sheep and nurses. The SNP has gone out of its way to court rural Scotland, including rural south of Scotland.

Yet even if there were to be some extra money available as the SNP contemplate putting up taxes, no-one thinks there is some magic money tree growing alongside the apples in the Holyrood parliament garden.

So if they get what they want on devolved farm payment powers ministers will still be faced with yet more of those famous 'tough choices' - see my earlier blogs.

And where would the public be in all of this? In the south of Scotland many might favour 'sheep' over 'nurses' - that's farming help over the NHS. Farming is a vital part of many communities.

But I suspect that across Scotland spending money on those who care for us might be more popular than on animals farmed for us.

Be careful what you wish for indeed.

You can see my interview with Prof Heald for last night's Representing Border here.

And if you would like more detail on this and other aspects of the spending challenges Holyrood's new tax powers present you can see Prof Head's paper to Holyrood's finance and constitution committee here.