With other phones, 3G-only and LTE versions were generally priced the same. If that's the case, you should go for the D6503:
• Resale: it will also appeal to people in LTE countries
• Future-proof: LTE could come to you before your next phone
If the radio is set to 2G/3G only, battery life SHOULD be the same between both models
Any ROMs SHOULD be supported in both.