It's a fair question but it's a matter of what you value the most I suppose. Here's my thinking:

Assuming that you get the both bonuses for the SPG conversion then you are getting:

20,000 SPG = 20,000 Avios + 5,000 (25% bonus) + 5,000 (conversion bonus)

= 30,000 Avios

So what's the value of 30,000 Avios and is it worth more than 20,000 SPG? So here's my analysis (I'll put the full calculations on my blog)

Based on purchase cost: Avios vs SPG

30,000 Avios costs 495 pounds to purchase

Purchasing 20,000 SPG points (the max) costs 595 dollars = 370 pounds pro-rataed for 30,000 SPG would be 560 pounds.

In this case, I would say that the argument for conversion wins.

However, the purchase cost value is largely academic. What's more important is what can you use the points / miles for?

Based on 'value' of points: Avios vs SPG

Clearly this is difficult to ascertain as the value depends on how you spend the points. I don't think there is a right or wrong answer but here's my thinking:

Avios (30,000) can buy:

2 x Zone 2 econ tickets e.g. London to Rome (random April date with sat stay)
Value of Avios = 66 pounds x 2 = 132 pounds

or

1 x Zone 2 business ticket e.g. London to Rome
Value of Avios = 464 pounds

While SPG (20,000) can buy:

One night in a category 6 hotel

e.g London W hotel on same random April night

Value of SPG points = 379 pounds

So in this example (and I'm sure there are plenty of other examples which would give different results depending on how you want to cut the cake), the business class ticket to Rome (Avios) trumps the hotel stay which trumps the 2 x econ tickets.

As such, the question remains which you would value more. Personally, I wouldn't pay 400+ pounds for a Club Europe ticket but understand that some people may choose that option over a hotel night.

Hope this helps

Jon

Full calculations on my blog