is the 64 bit or the 32 bit installer better? (not counting that if i have x>4 GByte RAM) - so i mean like is there any ASLR in OpenBSD?
So that 64bit would be advised to use?
On x86 processors, 64-bit mode offers significant performance benefits for some cryptographic operations, in particular asymmetric encryption and signatures; this is partly due to the bigger registers (it helps with big-integer arithmetics, and algorithms which rely on 64-bit operations) and due to the higher number of registers (that's a quirk of x86: 7 usable registers in 32-bit mode, 15 in 64-bit mode). It also gives a boost to floating-point, there again for historical reasons: a processor which knows 64-bit mode also knows SSE2, which is a set of opcodes which are more efficient for floating-point operations than the older 8087 FPU opcodes that are used in 32-bit mode.
Hence, on the x86 processors, 64-bit mode is good for performance (this performance boost more than makes up for the slight memory consumption increase due to the bigger pointers). The same would not apply on other 32/64-bit processors, e.g. PowerPC, where 32-bit mode is recommended for applicative code, except code which needs a very large address space.
OpenBSD implements ASLR. Note that ASLR is a damage-containment feature: it makes it more difficult to turn a buffer overflow into a remote shell exploit. However, the buffer overflow is still there; ASLR just means that most of the time, the attacker will "only" be able to crash the application. The benefit is, in my view, quite marginal.
Security-wise, one notable difference is that W^X can't work as well on i386 (32-bit) as it can on amd64 (64-bit). Some other arch have other benefits (for example sparc64 has StackGhost which adds more protection).
Apart from this, amd64 mode has more CPU registers, i386 mode has smaller addresses/data structures so you might make better use of the CPU's cache, so for a performance-sensitive application it's better to benchmark both modes.