you might not see it as being worth it swapping from your 20 now to a 25, but starting fresh, for the extra outlay the 25 would be worth it. $1000 for .5 sec off your e.t is extremely good value for money. and as you said yourself, both are nearly stock, every mod you add, the gap widens. so if your aiming for 250+rwkw, thats when you will see that its becomes worth it to swap from the 20 to the 25.
anyway, for the point of the thread, dont buy the other car and f*ck around doing more motor swaps, build a 30DET, you should still even be able to use everything you got now, ie loom/computer/manifolds (providing you use the right head), just build the motor, and go do some skids lol