In this scenario, it is better to use AES-128 than AES-256 if you are to 0-pad a 128-bit key to 256 bits.
If you 0-pad, the round key for round 1 is all 0s, and round 3 is effectively worthless as well. So now you are down to 12 effective rounds vs 10 for AES-128.
Then you need to look at the effectiveness of the remaining keys. Here are some example key schedules for AES-256 (0-padded 128-bit key) and AES-128:
00 a1b625fe 019e24c8 0a7f661b 9ad9d51c 00 a1b625fe 019e24c8 0a7f661b 9ad9d51c
01 00000000 00000000 00000000 00000000
02 c2d5469c c34b6254 c934044f 53edd153 01 3d0e10fc 3c903434 36ef522f ac368733
03 ed553eed ed553eed ed553eed ed553eed
04 9780ba2c 54cbd878 9dffdc37 ce120d64 02 fe9f15e9 c20f21dd f4e073f2 58d6f4c1
05 669ce9ae 8bc9d743 669ce9ae 8bc9d743
06 8dbd6726 d976bf5e 44896369 8a9b6e0d 03 86f5e352 44fac28f b01ab17d e8cc45bc
07 18887679 9341a13a f5dd4894 7e149fd7 04 e36ea834 a7946abb 178edbc6 ff429e7a
08 834e9df5 5a3822ab 1eb141c2 942a2fcf 05 3978842f 9eecee94 89623552 7620ab28
09 3a6d63f3 a92cc2c9 5cf18a5d 22e5158a 06 0d40336d 93acddf9 1acee8ab 6cee4383
10 fddd44bc a7e56617 b95427d5 2d7e081a 07 e1101b37 72bcc6ce 68722e65 049c6de6
11 e29e5351 4bb29198 17431bc5 35a60e4f 08 6fe2c58b 1d5e0345 752c2d20 71b040c6
12 794b6037 deae0620 67fa21f5 4a8429ef 09 db412299 c61f21dc b3330cfc c2834c3a
13 34c1f68e 7f736716 68307cd3 5d96729c 10 5b64ce86 9d7bef5a 2e48e3a6 eccbaf9c
14 a707f037 79a9f617 1e53d7e2 54d7fe0d
Round 5 is effectively 64-bits instead of 128, and while you cannot see it with this example, rounds 2 and 4 are also very linear with respect to subkey 0, and only at round 6 do we see a level of acceptable nonlinearity (we get this at round 3 with the 128-bit key schedule). Further nonlinear expansion is also slower, I would suggest the final round keys are similarly linear to the original key for both key schedules.
It is probable that even with the extra 4 rounds, the full AES-256 cipher with the padded key will be weaker than AES-128 against a variety of attacks, while taking 40% more time to encrypt. As CodesInChaos suggested, expanding a 128-bit key to 256-bits using a cryptographic hash will result in a stronger cipher than AES-128 while still only having an effective key size of 128-bits. I would not recommend duplication of the key to expand to 256-bits, round key linearity is still high, and it is probable existing attacks will be more effective.