Question about AMD APU+GPU

I'm not sure what you mean by "Dual Graphic" mode. The APU itself cannot use GDDR5 because it doesn't have any. It will use system memory.
If you are using the APU with a separate GPU, then they both will use their own memory-system memory for the APU and onboard GDDR5 for the GPU.

I don't think the better GPU with faster RAM would be a waste of money (depends on the price difference, of course). The GDDR5 version will have much better performance-perhaps even to the point that it will not benefit to use the APU at all. The benchmarks I have seen don't show much improvement using the dual graphics over just using a dedicated card. Either way, having a faster card can only help.l

Here are some benchmarks for you to consider:
http://www.techspot.com/review/781-amd-a10-7850k-graphics-performance/page5.html
As far as I can see, dual graphics actually lowers performance (sometimes) over just using the 250. If you can afford it, you would be much better off with a 260(x).

Let me separate my answer for clarity:
1. YES the GDDR5 version is better-I was only able to find specific benchmarks for the HD7750 (which is pretty much the same), but in practical benchmarks the faster memory can improve performance by 40%-50%. That faster performance can only help you.

2. I would like to believe that dual graphics can scale to 95%, but I was not able to find any benchmarks to confirm that. The only benchmarks I found that showed a net performance improvement of dual graphics over the r7 250 alone were ones that used the ddr3 version. If you take into account the performance boost of the faster memory, that performance delta would be completely erased. Dual graphics scales 95% above the APU performance, but it barely (if at all) scales above r7 250 performance. If you have any benchmarks that show that scaling, I would really like to see them.

3. This is a complicated issue and confusing. AMD's marketing only makes it more so. Here's my simplification that cuts through the obfuscation surrounding dual graphics: Dual Graphics scales better with weaker GPUs (240 and 250 ddr3), but the actual total performance is still lower (the 250 outperforms the 240 even though scaling is worse). The 250 gddr5 sits right at the tipping point where scaling decreases to the point where performance begins to regress. This is why they don't advertise dual graphics with any cards beyond the 250-it would actually lower performance.

4. The long and roundabout answer to your original question is that the ddr3 version will scale better, but the gddr5 version will give you better performance-so much better that it may be worth your while to not even attempt dual graphics.

5. I can't help but add that looking at Amazon to compare prices between the ddr3 and gddr5 versions I notice that not only is there not a price difference but you can actually pick up a 250x for less than the cost of the 250ddr3. The 250x will leave dual graphics in the dust. With a 250x you don't have to worry about frame pacing, which AMD has only mostly fixed, you don't have to worry about games that don't support multiple gpus, and you can still enjoy the benefits of using the APU graphics for compute tasks if AMD and developers ever get around to enabling that in actual programs/games.
http://www.amazon.com/MSI-Computer-R7-250X-1GD5/dp/B00JJNQ8S2/ref=cm_cr_pr_product_top?ie=UTF8
$70 after a $20 rebate or get the 2GB version for only $15 more.