You are poking around two of the all-time classics with the D18 and J45. What kind of music do you play? The D18 should be louder, brighter, and more articulate than the J45. The J45 will be easier to play (owing to the shorter scale length) and more percussive. It will also be quieter (again due to short scale) and might be easier to sing over / better for vocal accompaniment.
__________________
| 1968 Martin D-28 | 1949 Gibson J-45 | 1955 Gibson LG-2 | Santa Cruz 000 Cocobolo / Italian Spruce | Martin D-18 1939 Authentic Aged | Martin Gruhn Guitars Custom D-21 Adi/Madi | Gibson J-45 | Fender American Elite Telecaster | Fender American Standard Stratocaster | Gibson Les Paul Standard | Gibson Les Paul Studio | PRS Custom 24 10-Top | Gibson Les Paul 1960 Reissue (R0) |
|